As of Sunday afternoon, a meeting between U.S. President Barack Obama and Prime Minister Binyamin Netanyahu set for Monday is still on. Netanyahu left Israel on Sunday for Washington, where he is to meet Obama and speak at the AIPAC Conference on Tuesday. However, Israeli officials said that they were preparing for the possibility that Netanyahu’s meeting with Obama could be curtailed or cancelled altogether, given the way the Ukraine crisis is unfolding.

The crisis in Ukraine heated up Sunday afternoon, after reports said that hundreds of Russian soldiers had surrounded a Ukrainian military base in Crimea. The Russians were preventing the Ukrainians from leaving the base, with a tense standoff developing between the two sides.

In Kiev, Ukraine’s newly-installed Prime Minister Arseniy Yatsenyuk appealed to Moscow to move its troops back, saying that the region was “on the brink of disaster. There was no reason for the Russian Federation to invade Ukraine,” he said after meeting with Ukraine parliamentarians.

In response to the Russian moves, Ukraine ordered a full mobilization of its military. Soldiers are on “full combat readiness,” Kiev said, and the country’s Foreign Minister was appealing to US and European officials for defense assistance. Analysts said, however, that Ukraine’s forces were no match for Russia’s, and unless NATO or the Western powers were to get involved, it was unlikely that Ukrainian troops would be able to hold the Russians back.

Meanwhile, the BBC said Sunday afternoon that Russian soldiers were observed digging trenches on the border with Crimea, indicating that more active military action was possible. The UK announced Sunday that it was joining the US and Canada in pulling out of preparatory talks for the next G8 economic summit, which were to be held in Sochi, site of the recently-finished Winter Olympics.

U.S. President Barack Obama on Saturday threatened Russian President Vladimir Putin with political and economic isolation” over Russia’s military incursion in Ukraine. AFP reported that the warning came in a 90-minute phone call between the two leaders. Obama reportedly told Putin that Russia had violated international law with its incursion into Ukraine.

The White House said Obama called on Putin to pull Russian troops back to their bases in Crimea and warned that continued violation of international law by Moscow would lead to further “political and economic isolation.”