Russian President Vladimir Putin has said his country reserves the right to use all means to protect its citizens in eastern Ukraine, but added that the use of force would be the "last resort".

This developed as warning shots were fired on Tuesday when unarmed Ukrainian soldiers approached Russian forces guarding a seized airbase at Belbek near Sevastopol.

At a news conference on Tuesday, Putin said there had been an "unconstitutional coup" in Ukraine and that ousted leader Viktor Yanukovich was still the legitimate leader of the country despite giving up all power.

"The legitimate president, purely legally, is undoubtedly Yanukovich," Putin said.

He said this meant Russian intervention in Ukraine would be justified because Moscow had received a request for protection of its citizens from Yanukovich.

He also said that troops in Crimea had been invited in, and were not Russian soldiers, but pro-Russian self-defence forces.

"We have a direct request from the acting and legitimate - as I have already said - president Viktor Yanukovich about using armed forces to protect the lives, health and freedom of Ukrainian citizens," Putin said.

"We reserve the right to use all available means. And we believe that this is fully legitimate. This is a last resort."

The comments were Putin's first public reaction to the crisis in Ukraine since the ousting of Yanukovich.

Also on Tuesday, US secretary of state, John Kerry, arrived in Kiev to support the country's new government, while stepping up pressure on Moscow to withdraw its troops from Crimea, calling it an occupation.

The US government also announced that it would work to get $1bn in loans to help lessen the impact on Ukranians of Russia proposing to life subsidies on the cost of energy it supplies.

The United States will also send technical experts to advise Ukraine's central bank and finance ministry on how to deal with the country's economic challenges and help combat corruption, according to the Reuters news agency.

In addition, the US will train observers for the country's May 25 election to bring the electoral process in line with international standards.

'No political future'

Putin told the news conference that Yanukovich had no political future and that Russia had sheltered the 63-year-old leader "to save his life".

The West is facing its biggest confrontation with Moscow since the Cold War/ Ukraine has put its military on alert.

Kerry and other top diplomats have repeatedly warned Russia about the consequences of intervention. Overnight on Monday, the US announced it was suspending military exercises and trade talks with Russia.

But Putin said that sanctions would affect other economies too. "Those considering sanctions should think about the consequences ... in this tightly connected world we can of course harm each other but it will be mutual harm, and they need to think about that. As far as the G8, we are preparing for the summit, we are ready to host our colleagues if they come, but they don’t have to."

He said that Russia was prepared to financially assist Ukraine but that late gas payments were preventing it from doing so. If Ukraine fails to pay for its February gas supplies, it will owe Russia $2bn.