WASHINGTON — President Barack Obama said on Monday that Russia has violated international law with its military intervention in Ukraine and warned that the U.S. government will look at a series of economic and diplomatic sanctions that would isolate Moscow.

Russian President Vladimir Putin needs to allow international monitors to mediate a deal in Ukraine acceptable to all Ukrainian people, Obama told reporters before he met with Israeli Prime Minister Benjamin Netanyahu.

The U.S. is weighing its response to Russia’s so far bloodless incursion into Crimea. Despite an international outcry, Putin has shown little sign of backing down, and Russia has built up its armored vehicle presence near Crimea and staged military maneuvers in what appears to be a show of strength.

“Over time this will be a costly proposition for Russia. And now is the time for them to consider whether they can serve their interests in a way that resorts to diplomacy as opposed to force,” Obama said.

U.S. Secretary of State John Kerry will propose ways in which a negotiation between Russia and Ukraine could be overseen by a multilateral organization when he goes to Kiev on Tuesday, Obama said. Kerry also will offer an economic aid package for Ukraine, the president added.

In the meantime, the United States is preparing to impose sanctions on Russia over the intervention although no decisions have yet been made, the State Department said. State Department spokeswoman Jen Psaki said the United States has a broad range of options.

In her daily briefing, Psaki also said that if reports that Russia has threatened Ukraine’s military in Crimea with attack are true, this would be a “dangerous escalation” of the situation.

As the administration sought to rally international pressure against Russia, U.S. lawmakers were considering imposing sanctions on Russia’s banks and freezing assets of Russian public institutions and private investors in response to events in Ukraine, one senator said.

The lawmaker, Democrat Chris Murphy, warned that in order to be effective, U.S. sanctions would need to be matched by similar moves by Europe.

“If the United States shuts its economic doors to Russia and Europe leaves its doors open, there won’t be much change in behavior from Moscow,” he told Reuters.

Sen. Susan Collins, R-Maine, condemned Russia’s actions.

“I condemn Russia’s invasion of Ukraine, and join the Administration and our European allies in calling for Russia to withdraw its soldiers and refrain from further violations of Ukraine’s sovereignty and international law. I support the cancellation of upcoming G-8 summit in Russia until such time as Russia is willing to stop its meddling in Ukraine and act in a manner that is consistent with the principles and values of the G-8,” Collins said. “I stand ready to work with my colleagues in Congress and the Administration to support Ukraine through loan guarantees to help stabilize Ukraine’s economy and through targeted sanctions applied to Russian officials to discourage Russia from further undermining Ukraine’s territorial integrity.”

Obama has been criticized by opposition party Republicans for not being strong enough in his response to the Ukraine crisis.

The president called on lawmakers to unite behind an aid package for the embattled eastern European country.

“When it comes to preserving the principle that no country has the right to send in troops to another country unprovoked, we should be able to come up with a unified position,” he said.

At the Obama administration’s request, members of both the Senate and House of Representatives are working on legislation to support U.S. loan guarantees for Ukraine, congressional aides said.