Ukraine PM Yatsenyuk in Russia border demand
Ukrainian Prime Minister Arseniy Yatsenyuk has called on Russia to control its border to stop "terrorists" from crossing into his territory.
Mr Yatsenyuk said Kiev could solve the crisis quickly if Moscow stopped meddling in the situation.
But Russian Foreign Minister Sergei Lavrov warned that Kiev's policies were pushing Ukraine into "fratricidal war".
Earlier this week rebels and government forces were embroiled in some of the worst fighting of the crisis so far.
The separatists say they lost up to 100 fighters as they tried to seize Donetsk airport from pro-Kiev forces on Monday.
Ukraine's interior ministry says the military is now in full control of the airport.
On Wednesday, gunfire was reported in Donetsk city and rebels attacked a military base in the city of Luhansk causing casualties on both sides.
Speaking on a visit to Germany, Mr Yatsenyuk said the situation in the east was deteriorating and Russia's involvement was causing huge difficulties.
"A number of trucks full of live ammunition, full of Russian-trained guerrillas crossed the Russian border into Ukraine," he said.
"We ask Russia and Putin to block the border to Ukraine. If Russia is out of this game we can handle this situation in a week."
Meanwhile, the Organization for Security and Co-operation in Europe (OSCE) said it had re-established contact with a monitoring team it reported lost in eastern Ukraine on Tuesday, but it continued to refer to the group as "detained" and their fate is unclear.
UN Secretary General Ban Ki-moon has called newly elected President Petro Poroshenko to congratulate him on receiving a "strong mandate" to govern after a landslide election victory on Sunday.
Mr Poroshenko said he would hold direct talks with Russian counterpart Vladimir Putin as soon as possible to defuse the crisis.
His office also said he would seek to sign a trade deal with the EU as soon as possible - a move likely to anger Moscow.
The election had been criticised because many in the east were unable to vote as a result of the unrest.
But analysts say the wide margin of Mr Poroshenko's victory has bolstered the confidence of Ukrainian officials in their dealings with Moscow.
On Tuesday, Mr Yatsenyuk denied Moscow's long-standing claim that Ukraine owed billions of dollars in unpaid gas revenues.
The prime minister said that in fact Russia's state-owed Gazprom owed Ukraine $1bn (£600m) in compensation for natural gas seized in the annexation of Crimea.
Gazprom's chief executive Alexei Miller said on Wednesday that Ukraine had used $1.7bn worth of gas in May alone, and would owe $5.2bn by 7 June.
He demanded pre-payment for June's gas supply, and threatened to restrict exports if money was not transferred by 3 June.