KIEV, Ukraine — As Ukrainian officials prepared to campaign in the United States this week for more international support ahead of a Russian-backed referendum on secession in Crimea, Moscow complained Monday of “lawlessness” in eastern Ukraine, raising fear it might widen its military intervention to include that region.

The Kremlin said in a statement that Ukrainian right-wing extremists, taking advantage of the “complete neglect” of the new Western-oriented government in Kiev, were threatening order in eastern Ukraine. In addition, the statement said, Russian citizens trying to cross the border into Ukraine were being turned back by Ukrainian border agents.

The allegations added to the increasingly heated rhetoric flying between Kiev and Moscow, and sparked concern that the Kremlin was setting up a pretext for a new military incursion. President Vladimir Putin has justified aggressive moves by pro-Russian forces in Crimea, in southern Ukraine, on the grounds of needing to protect ethnic Russians on the strategically valuable peninsula, though no independent group has identified any instances of danger or abuse.

In a sign of increasing Western concern, NATO said it was sending AWACS surveillance aircraft over Poland and Romania on reconnaissance missions to montor the situation in Ukraine. The NATO planes will be dispatched from air bases in Germany and Britain, a spokesman said on customary condition of anonymity.

Crimea’s Russian-backed assembly has scheduled a referendum for Sunday on splitting off from Ukraine and joining Russia. Western nations, including the U.S., have denounced the move as illegal.

Hoping to rally more diplomatic support, acting Ukrainian Prime Minister Arseny Yatsenyuk is due to meet President Obama at the White House on Wednesday for talks on the situation and on how to shore up his country’s flailing economy. Yatsenyuk is also planning to address the U.N. Security Council on Thursday. He told reporters in Kiev that there was still space for a diplomatic solution to the crisis in Crimea but accused Moscow of spurning peaceful political negotiations.

Russian Foreign Minister Sergei Lavrov said U.S. Secretary of State John F. Kerry had postponed a planned trip to Moscow on Monday, according to a statement on the Foreign Ministry’s website.

Lavrov said an American proposal for resolving the Crimean crisis “doesn’t suit us very much because everything in it was formulated as if there were a conflict between Russia and Ukraine.” Putin denies having sent in troops to take over the Crimean peninsula, saying that local pro-Russian forces have seized control of state and military installations on their own initiative, even though the armed men wear Russian military uniforms, use Russian military vehicles and have, on occasion, openly acknowledged being Russian soldiers.