Pro-Kremlin militia fired warning shots as unarmed foreign observers tried to enter Crimea on Saturday while Russia stepped up its standoff with the West over Ukraine by threatening to keep Washington from checking on its nuclear arms.

The dangerous escalation in the Black Sea peninsula that Russia effectively seized last week saw 40 gunmen in balaclavas and military fatigues fire shots above a convoy with 54 civilian and military observers from 29 countries of the Organisation of Security and Co-operation in Europe (OSCE).

It was the third successive day that the OSCE buses attempted to enter the tension-wracked peninsula to check on the frozen conflict that has pitted overpowered Ukrainian troops against Russian forces and Kremlin-backed militias.

Ukraine's border guards service added to the tinderbox atmosphere by reporting that one of its light patrol planes came under fire from "extremists" in the same area but was unscathed.

The monitoring mission is an instrumental part of a three-pronged diplomatic push by US President Barack Obama that also includes a call on Russia to pull back its Crimean troops to their barracks and for Ukraine to hold early presidential polls in May.

The culturally splintered ex-Soviet nation of 46 million has been in upheaval since three months of deadly unrest brought new, pro-European leaders to power in Kiev in February and sent ousted president Viktor Yanukovych into hiding in Russia.

The Kremlin accuses the new rulers of fomenting an atmosphere of intimidation against the Russian speakers who make up the majority of Ukraine's southeast. That prompted President Vladimir Putin to threaten to use force -- a shock decision that has sparked the worst East-West security crisis since the 1989 fall of the Berlin Wall.

Russia says it has stepped up protection of its Crimean naval base and is working together with "local self-defence units", but refuses to acknowledge deploying extra troops.

But Ukrainian Border Guards General Mykola Kovil said there were now 30,000 Russian soldiers in Crimea -- 5,000 more than the contingent allowed under an agreement with Kiev linked to Russia's Black Sea Fleet base.

The border guards' service on Saturday also accused Russia of seizing a ferry terminal and a navy yard in the east and southwest of Crimea in order to send convoys of 67 military transport trucks filled with soldiers toward the region's inland capital Simferopol.

- Russia raises nuclear threat -

Russia's decision to flex its military muscle has prompted Washington to announce plans to introduce visa bans and asset freezes on Moscow officials while also halting various forms of military cooperation.

The European Union has further threatened to toughen economic sanctions against Russia should it fail to immediately open talks with Ukraine.

The office of French President Francois Hollande said he and Obama agreed by telephone on Saturday that "new measures will be taken which would noticeably affect relations between the international community and Russia" should Moscow fail to quickly defuse the escalating security threat on Europe's eastern edge.

But Russian Foreign Minister Sergei Lavrov told US Secretary of State John Kerry that any punitive measures would strike back like a "boomerang".

Those warnings became more concrete on Saturday when a Russian defence official said Moscow may halt foreign inspections of its vast nuclear arsenal in response to "threats" from the United States and the North Atlantic Treaty Organisation (NATO).

The arms inspections are carried out in line with the historic 2010 Strategic Arms Reduction Treaty (START) -- a highlight of Obama's diplomacy -- and the Vienna Document of 2011 on confidence-building measures between OSCE member states.

The state-run energy giant Gazprom had begun to pile the pressure on Friday by warning Ukraine it may cut off gas shipments due to accumulating debts.

The halt would almost certainly affect Gazprom's Western European clients and further damage the Kremlin's reputation.

Russia's largest company is often accused of being wielded by the Kremlin as an economic weapon against uncooperative neighbours that are considering breaking ties with Moscow and, consequently, hindering Putin's dream of building post-Soviet alliances to rival the NATO and EU blocs.

- Media and cyber warfare -

The military standoff and diplomatic wrangling have translated into an information war. Western and Ukrainian journalists have been threatened by Crimean gunmen and the peninsula has replaced broadcasts from several television channels based in Kiev with Russian ones.

OSCE media freedom representative Dunja Mijatovic called on Crimea's pro-Kremlin authorities to do the "utmost to ensure safety of journalists".

Britain's BAE Systems defence and security company meanwhile reported that Ukraine has been stricken by a powerful computer virus whose likes had not been seen since the Stuxnet malware programme wreaked havoc on Iran's uranium enrichment system in 2010.

The British defence group said a code in the virus suggested it was created in the same timezone as Moscow.

- Pro-Russian protests -

The Crimean parliament's decision to secede from Ukraine and stage a March 16 referendum on joining Russia has been also been picked up in industrial eastern regions such as Yanukovych's home base of Donetsk.

"We want to join Russia because we know their standard of living is well above what we have here," said retiree Larisa Kukovina as she and a thousand other mostly elderly protesters faced off against 500 officers who had encircled the Donetsk administration building.

The seat of the region's power has alternately flown Russian and Ukrainian flags after being repeatedly raided in the past week by protesters from both sides.