TRIPOLI, Libya — Moammar Gadhafi’s government has promised the U.N. access to the besieged rebel city of Misrata, a senior U.N. official said Monday, even as Libyan government troops continued to pound opposition enclaves with shells and rockets.

Libyan authorities, however, have not guaranteed a halt of hostilities during such a mission, said the U.N.’s humanitarian chief, Valerie Amos, who met with the Libyan prime minister in the capital Tripoli on Sunday.

Amos said she will try to send a team to the city of 300,000 as quickly as possible, adding that she’s “deeply concerned” about the safety of civilians there.

The Libyan government denies it’s using heavy weapons in Misrata, a rebel bridgehead in Gadhafi-controlled western Libya. However, Rida al-Montasser, a Misrata activist, said Gadhafi forces continued to fire rockets and tank shells into the city on Monday as rebels battle for control of the urban core.

Residents and hospital officials in the city have described heavy shelling over the weekend, and said 17 people were killed Sunday. U.N. officials said children and elderly people have been among the casualties in recent days.

U.N. Secretary-General Ban Ki-moon appealed to Libyan forces to hold their fire. “Considering the magnitude of this crisis, and as this fighting is still continuing, it is absolutely necessary that Libyan authorities stop the fighting, stop killing people,” he told a news conference in Budapest, Hungary.

Ban said the basic needs of tens of thousands of people in Libya are not being met.

In Germany, Economics Minister Rainer Bruederle suggested that the Libyan government’s frozen assets could fund the U.N. humanitarian mission in Libya. But a German official, speaking on condition of anonymity citing the sensitivity of the issue, said any such decision on the assets required “a multilateral, international agreement.”

Gadhafi’s government has said about $120 billion in assets have been frozen worldwide.

The U.N. has already set up an aid operation in Benghazi, the rebel stronghold in the eastern part of Libya they control. As part of Sunday’s agreement with the Gadhafi government, the U.N. would be allowed to set up a presence in Tripoli as well, Amos said, speaking in Benghazi.

She said in her meetings in Tripoli on Sunday, she pressed the government, “particularly in relation to Misrata, but also in other vulnerable towns and cities, and to seek their assurances that we could carry out an independent needs assessment of the situation in Misrata,” she said. “I have been given those assurances.”

However, she added that she received “no guarantees with respect to my call for an overall cessation of hostilities, to enable people to move, to enable us to deliver supplies.”

Libyan government spokesman Moussa Ibrahim said the deal with the U.N. includes setting up a humanitarian corridor to Misrata and access for international aid agencies.

“The agreement is to provide safe passage for people to leave Misrata, to provide aid, food and medicine,” Ibrahim said late Sunday. He did not say whether this would include a halt in fighting. The Libyan authorities have insisted they are only responding to attacks by rebels, but not taking the offensive.

Ibrahim said the agreement with the U.N. also included ensuring that electricity, water and other services are provided to Misrata. City residents have said supplies have been severely disrupted.

Al-Montasser said he was concerned that aid personnel could be in danger if Gadhafi forces are still in the city.

He said rebels and pro-Gadhafi units battled over the city center, after rebels managed to drive them out. On Monday, Gadhafi forces tried to retake the area, and rebels fought them off again.

Early Monday, nearly 1,000 people — among several thousand stranded in the area of Misrata’s port — boarded an aid ship sent by the International Organization for Migration.

Most of the passengers were migrant workers, but also included 100 Libyans, among them 23 wounded in the fighting. The injured included a child shot in the face and an amputee, the aid group said.

“We wanted to be able to take on more people, but it was not possible,” said Jeremy Haslam, who heads the group’s boat rescue. “Although the exchange of fire subsided while we were boarding with an eerie silence at one point, we had a very limited time to get the migrants and Libyans on board the ship and then leave,” he said.

The organization said at least 4,000 additional migrants are stranded in the port area, including women and children.

Many of the refugees have been living out in the open or in containers in the port area for nearly two months, lacking access to water and medical and running short on food. IOM said many of the migrants are weak and dehydrated.

Haslam said the group needs funding for a bigger boat to rescue the remaining migrants in one trip.

Al-Montasser said other migrant workers are now making their way to Misrata from nearby cities, hoping to be evacuated.

Libya’s fighting, which erupted two months ago, has reached a deadlock, with neither side able to gain a decisive advantage and the front line shifting back and forth across a stretch of desert near the eastern city of Ajdabiya.

On Monday, rebel fighters and Gadhafi’s forces battled for a second consecutive day around Ajdabiya, exchanging light volleys of artillery and rocket fire near the city’s western gate.

One of the rebels’ staunchest supporters is the gulf state of Qatar, which has given them diplomatic and financial backing. Late Sunday, the leader of the rebels’ transitional government, Mustafa Abdul-Jalil, arrived in Qatar, the official Qatar News Agency reported. Abdul-Jalil was met by Qatar’s foreign minister.

On Tuesday, Abdul-Jalil is expected in Rome for talks with the country’s top officials, including Prime Minister Silvio Berlusconi and Foreign Minister Franco Frattini. Earlier this month, Italy recognized the rebels’ transitional government as the only legitimate representative of the people of Libya.

“Italy is extremely worried over the civilian population in Libya and especially in Misrata,” Foreign Ministry spokesman Maurizio Massari said on the eve of the meeting.

NATO, meanwhile, said it needed nine more attack jets to launch raids in Libya, but it is confident it would obtain them. NATO has deployed nearly 200 aircraft for the operation, which started almost a month ago with airstrikes by a U.S.-led coalition of countries. On Monday, Libyan state TV reported NATO airstrikes near the town of Aziziyeh, south of Tripoli, but provided no details.

In Brussels, NATO said it can’t confirm hitting any specific targets until they receive mission reports, but noted that the attacks have been occurring “both in daytime and nighttime.”

NATO, the North Atlantic military alliance, assumed control of the mission at the end of March. Since March 31, NATO has carried out more than 2,770 sorties over Libya.

A statement said alliance jets flew 145 sorties on Sunday, including 60 strike sorties. They destroyed nine ammunition storage depots and bunkers, two hangars, five air defense radars, and four missile launchers.

Associated Press writers Pablo Gorondi in Budapest, Hungary; Sebastian Abbot in Ajdabiya, Libya; Juergen Baetz in Berlin and Alessandra Rizzo in Rome, and Slobodan Lekic in Brussels contributed to this report.