John Kerry, US secretary of state, has emphasised America’s readiness to facilitate dialogue between Ukraine and Russia but warned that continued military escalation or moves to annex Crimea “would close any available space for diplomacy”, according to a senior US official.

His warning comes as Ukraine on Saturday suggested that international efforts to mediate in the dispute over Crimea had made “some small” progress, despite evidence of increasing tensions on the peninsula and in east-west relations. There were no reports of clashes in Crimea overnight.

Andriy Deshchytsia, Ukraine’s acting foreign minister, said there had been “some small steps” forward in efforts to establish a contact group of Ukraine, Russia and some western countries.

However, Mr Deshchytsia declined to give details, saying that even the group’s membership had yet to be set and progress was “fragile”. He also insisted Ukraine remained opposed to Crimea’s plans to hold an “illegal” referendum next Saturday on whether to declare independence or join Russia.

In Moscow, Russia’s deputy foreign minister met the Ukrainian ambassador to Moscow, where they had a “frank” discussion on relations between the two countries, according to the Russian foreign ministry. Sergei Lavrov, Russia’s foreign minister, also spoke to John Kerry, his US counterpart.

In a sign of how the dispute is causing damage to east-west relations, the Russian defence ministry was reported to be considering suspending US military inspections under START, the strategic arms reductions treaty, and the Vienna Document of 2011.

“The unjustified threats to Russia on the part of the US and Nato with regard to its Ukrainian policy are seen by us as an unfriendly gesture,” Russian news agencies quoted a high-ranking official in the Russian defence ministry as saying.

Given the US sanctions on Russia, there could be “no normal bilateral contacts”, the agencies quoted the official saying.

Mr Lavrov also stuck to Moscow’s tough line, saying on Saturday that the crisis in Ukraine was “created artificially for purely geopolitical reasons”.

But, in comments that seemed to suggest that there had been movement on the diplomatic front, he confirmed Russia had achieved contacts with Ukraine’s interim government and that Russia was open to further dialogue with the west if it was “honest and partner-like”.

Barack Obama, US president, discussed the formation of a contact group on separate calls with Britain’s David Cameron, France’s François Hollande, and Italy’s Matteo Renzi, and on a conference call with the presidents of Latvia, Lithuania and Estonia.

However, there remain huge difficulties in developing the contact group plan, first proposed by Angela Merkel, German chancellor, in a phone call to president Vladimir Putin just after Russian forces began their de facto invasion of Crimea last weekend.

Ukraine insists that before talks start, Moscow must pull its troops out of Crimea and respect international agreements.

Russia is threatening to retaliate against the US decision to impose a visa ban on some top officials and consider financial sanctions.

The EU has approved a political association deal with Kiev that is a key part of the planned wide-ranging political and economic pact that Russia sees as hostile.

The EU is also considering following the US with sanctions if the situation on Crimea escalates. A senior UK official yesterday suggested that the proposed referendum could be such an escalation.

Meanwhile, a group of military observers from the OSCE, a European inter-government security body, was prevented for a third time from entering Crimea. The 40-plus unarmed monitors were turned back after troops believed to be Russian fired warning shots at them. No one was hurt.

Also on Saturday, local media reported seeing a convoy of about 50 trucks carrying hundreds of Russian troops, accompanied by armoured vehicles and ambulances. Reuters reported that the convoy, which had come from the direction of Feodosiya on the western coast of Crimea close to Russia, pulled into a military base near the Crimean capital of Simferopol.

Elsewhere, Ukraine was mostly calm, despite growing concerns about the safety of troops surrounded by Russians in bases in Crimea.

In eastern Ukraine, there were modest demonstrations in Kharkiv and Donetsk of critics of the Kiev interim government. In contrast to Crimea, the protesters did not call for union for Russia but for stronger Russian language rights and more regional autonomy within Ukraine. In Kharkiv, some demonstrators waved Ukrainian passports to prove they were locals, and not Russians bussed in by agents provocateurs.

Additional reporting by Geoff Dyer in Washington

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