US Secretary of State John Kerry has told Russian Foreign Minister Sergei Lavrov that any Russian steps to annex Crimea would close the door to international diplomacy.

Mr Kerry also urged Mr Lavrov to ensure that Russia acts with the "utmost restraint" in Ukraine.

It follows warnings from French President Francois Hollande and US President Barack Obama that "new measures" would be implemented against Russia if it fails to move on defusing the crisis in Ukraine.

The two leaders insisted on the "need for Russia to withdraw forces sent to Crimea since the end of February and to do everything to allow the deployment of international observers".

"If there's a lack progress in this direction, new measures will be taken which would noticeably affect relations between the international community and Russia, which is in no-one's interest," the French presidency said.

Mr Obama also spoke with British Prime Minister David Cameron and Italian Prime Minister Matteo Renzi in relation to Ukraine.

He also reaffirmed Washington's "unwavering commitment" to its NATO defence commitments in a conference call with Baltic leaders.

The crisis in Ukraine, which resulted in the ouster last month of pro-Russian president Viktor Yanukovych, has worsened since the Crimean parliament's decision to secede from Ukraine.

It plans to stage a referendum on joining Russia on 16 March.

Ukraine said there are now 30,000 Russian soldiers in Crimea - 5,000 more than the contingent allowed under an existing agreement with Kiev.

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

"In the current grave circumstances," Mr Hollande and Mr Obama "stressed the importance for Russia to agree rapidly to the formation of a contact group allowing for Ukraine and Russia to engage in dialogue, with a view to favouring a peaceful exit to the crisis and restoring fully Ukraine's sovereignty and territorial integrity," a statement from the French presidency said.

"They recalled the absence of any legal basis to the planned referendum in Crimea on March 16," it added.

The two leaders also agreed to continue their support for the new pro-western authorities in Ukraine as well as for the preparation for presidential elections on 25 May, "under international control and in the greatest transparency".

The US has already imposed visa bans and set the stage for wider sanctions against Russia over the seizure of the Ukrainian region of Crimea by pro-Russia forces.

Russia in warning over US sanctions

Earlier, Russia said any US sanctions imposed on Moscow over the crisis in Ukraine will boomerang back on the United States.

The Kremlin also insisted that Crimea has the right to self-determination as armed men tried to seize another Ukrainian military base on the peninsula.

In a telephone conversation with Mr Kerry, Mr Lavrov warned against "hasty and reckless steps" that could harm Russian-American relations, the foreign ministry said.

"Sanctions ... would inevitably hit the United States like a boomerang," it added.

Mr Kerry stressed the importance of resolving the situation through diplomacy and said he and Mr Lavrov would continue to consult, the State Department said.

Russia’s foreign ministry has also responded angrily to European Union’s three-stage plan to try to force a negotiated solution, which stop short of immediate sanctions.

The EU, Russia's biggest economic partner and energy customer, has frozen talks on visa-free travel and on a broad new pact governing Russia-EU ties.

Russian officials called this “extremely unconstructive” and pledged to retaliate.

Ukraine stands firm over Crimea

As the crisis continues, Ukraine's acting foreign minister said his country would not give up Crimea and would do all in its power to resolve the crisis over the Black Sea peninsula peacefully.

"Crimea is and will be Ukrainian territory and we will not give up Crimea to anyone," Andriy Deshchytsia told a news conference in the Ukrainian capital Kiev.

Referring to deaths this year during protests against Mr Yanukovych, he said: "We are putting all our efforts into solving this matter through diplomacy - we have already had too many victims."

The Organisation for Security and Co-operation in Europe has said that warning shots were fired as monitors approached an entry point into Crimea.

A source said "probably three shots" were fired as a convoy, including the observers' buses, approached a checkpoint manned by pro-Russian forces, but added the shots did not appear directed at the observers.

Russia moves more troops into Crimea

Elsewhere, a convoy of hundreds of Russian soldiers in about 50 troop trucks drove into a base near Crimea's capital Simferopol.
The convoy was accompanied by eight armoured vehicles, two ambulances, petrol tankers and other hardware.
Russia says its only troops in Crimea are those normally stationed there with its Black Sea Fleet, an assertion Washington calls "(President Vladimir) Putin's fiction".

Mr Lavrov has said the Ukrainian government was taking orders from extremists and denied Moscow had any direct role in the crisis in Crimea.

"The interim government... is not independent. It depends, unfortunately, on radical nationalists who carried out an armed coup," he told a news conference in Moscow.

The head of Russia's upper house of parliament, after meeting visiting Crimean politicians yesterday, said that Crimea had a right to self-determination, and ruled out any risk of war between "the two brotherly nations".

However, yesterday’s incident has led Poland to evacuate its consulate in Sevastopol, according to Polish Foreign Minister Radoslaw Sikorski.

Mr Sikorski said on Twitter: "Because of continuing disturbances by Russian forces there, we have reluctantly evacuated our consulate in Crimea, Ukraine."

Senior Ukrainian opposition politician Yulia Tymoshenko, freed from prison after Viktor Yanukovych's overthrow, met German Chancellor Angela Merkel in Dublin and appealed for immediate EU sanctions against Russia, warning that Crimea might otherwise slide into war.

Brussels and Washington rushed to strengthen the new authorities in economically shattered Ukraine, announcing both political and financial assistance.

The regional director of the International Monetary Fund said talks with Kiev on a loan agreement were going well and praised the new government's openness to economic reform and transparency.