Ukraine's acting president has called for the resumption of military operations against pro-Russian separatists in the east of the country, claiming two of his party's supporters had been "tortured to death", in a further blow to an unravelling international peace plan.

Oleksandr Turchynov said "counter-terrorist" operations in the region, suspended as part of the peace agreement in Geneva last Thursday, should restart after the bodies of two men, one a pro-Kiev politician, had been found near the rebel-held town of Slavyansk.

The politician has been named as Vladimir Rybak, a town councillor and member of the Batkivshchyna party, who went missing on 17 April.

"The terrorists who effectively took the whole Donetsk region hostage have now gone too far, by starting to torture and murder Ukrainian patriots. These crimes are being committed with the full support and connivance of the Russian Federation," Turchynov said, hours after a joint appearance with the US vice-president, Joe Biden.

The country's defence ministry also reported that one of its observation planes had been struck by gunfire from Slavyansk, one of the areas of greatest tension in the eastern region. The plane landed without injuries, the ministry added.

Turchynov's call to relaunch army operations came on a day when international monitors reported a worsening in the security situation in separatist-held eastern districts, while the US and Russia blamed each other for the continuing unrest.

Biden flew to Kiev to offer the Ukrainian government economic support and tell Moscow it was "time to stop talking and start acting". In response, Russian foreign minister Sergei Lavrov said the onus was on Washington to rein in the authorities in Kiev, which he said had been brought to power by the US and was responsible for "outrages".

Western officials acknowledged that the Geneva plan – agreed on Thursday by the US, Russia, Ukraine and the EU – was clearly in trouble, but the US and the EU put off a decision on imposing new sanctions on Russian leaders, hoping diplomacy could somehow be salvaged in the new few days.

"The negative rhetoric we have seen coming from Moscow is not what we expected from Russia, but we are going to give some time to this, while we make it clear to Russia there needs to be movement," a western diplomat said, adding that a further Geneva meeting to narrow differences could not be ruled out.

On the ground, the Organisation for Security and Cooperation in Europe (OSCE), tasked with monitoring and helping implement the Geneva plan, reported very little progress over the weekend towards two of its main goals: disarming rival groups in eastern Ukraine or ending the occupation of public buildings by pro-Moscow militias.

Meanwhile, the OSCE said the security in the most troubled areas was getting worse. An American journalist working for Vice News, Simon Ostrovsky, was reported to have been held by the separatists running Slavyansk, on the orders of their leader, the self-styled "People's Mayor", Vyacheslav Ponomaryov.

Amid a growing number of reports of abductions, arrests and disappearances, the head of the OSCE mission, Ertugrul Apakan, called for pro-Russian separatists to release the chief of police in the eastern town of Kramatorsk, Colonel Vitaliy Kolupai. "The OSCE monitors talked to witnesses who were able to confirm reports that armed individuals who called themselves supporters of the so-called 'Donetsk Republic' entered the premises of Kramatorsk police department and abducted its head Colonel Vitaliy Kolupai who is being kept against his will," the Vienna-based organisation said in a statement.

The OSCE added that because of "unpredictable security risks", its observers had been unable to reach the site of a shooting on Sunday morning in another separatist-held town of Slavyansk, in which at least three people were reported dead. Moscow and its supporters in the region blamed Ukrainian rightwingers. Kiev blamed Russian military intelligence.

The OSCE mission called the incident "a worrying deterioration of the situation" and said Slavyansk had become a no-go zone. "The security situation is assessed as deteriorating, and operating conditions for OSCE teams are marginal," it said.

Elsewhere in the region, the 150-strong monitoring team "did not find any indications that the Geneva statement was starting to be implemented in the Luhansk region" where pro-Moscow activists refused to move from government buildings they have been occupying for two weeks and instead announced a two-stage referendum on independence from Ukraine and union with Russia. In Donetsk, the OSCE reported that "occupation of state institutions was ongoing".

Vice-president Biden arrived in Kiev with offers of an extra $8m in non-lethal military assistance for the Ukrainian armed forces, including bomb-disposal equipment, communications gear and vehicles. He also pledged $50m (£30m) to help Ukraine's government to carry out political and economic reforms, including $11m to help conduct the presidential election on 25 May, but said the acting leadership must "fight the cancer of corruption that is endemic in your system".

Speaking in Kiev, Biden called on Russia to persuade its supporters in eastern Ukraine to disarm and to pull back troops from the Ukrainian borders.

"We've heard a lot from Russian officials in the past few days. But now it's time for Russia to stop talking and start acting," he told a news conference. "We will not allow this to become an open-ended process. Time is short in which to make progress."

Lavrov, however, shrugged off the US pressure, and argued that it was the US-backed government in Kiev that was destabilising the situation.

"Before putting forth ultimatums to us, demanding fulfilment of something within two-three days or otherwise be threatened with sanctions, we would urgently call on our American partners to fully recognise responsibility for those whom they brought to power and whom they are trying to shield, closing their eyes to the outrages created by this regime and by the fighters on whom this regime leans," Lavrov said.