Senior US Republican senators John McCain and Lindsey Graham have arrived in Estonia on on a visit seen as a bid to reassure Baltic states concerned that President-elect Donald Trump may not be fully committed to their defence.

Mr Trump unnerved many in Estonia, Lithuania and Latvia by saying on the campaign trail he would consider a country's contributions to the NATO alliance before coming to its aid.

Russian military involvement in Ukraine and Georgia has stoked fears in the Baltics their former Soviet master might eventually try something similar there.

"I think this visit is being done to emphasise that, whatever happens after the inauguration, the US Senate will be something the Baltic states can calmly rely upon," Zygimantas Pavilionis, Lithuania's former ambassador to the US, told Reuters.

Mr McCain and Mr Graham, who travel to Latvia tomorrow and Lithuania on Thursday, will meet heads of state and top defence officials.

The two, seen as defence policy hawks, did not back Mr Trump's presidential bid and expressed alarm at his attitude towards Russia.

Ojars Eriks Kalnins, head of the Latvian parliament's Foreign Affairs Committee, said the senators had expressed strong support at a meeting in Washington earlier this month.

"I think it is very clear to us that both Republicans and Democrats are still firmly committed to Baltic States and to NATO and to our security," Mr Kalnins said.

Russia's annexation of Crimea in 2014 fuelled concerns in the Baltic states, all three of which are NATO members, thatMoscow may want to reassert control across the whole region.

"There is fear in the Baltics about the incoming Trump administration's relationship with Russia, that sanctions against Russia will be weakened or called off, and notstrengthened as the Congress would want," Mr Pavilionis said.

He said Russia had gained confidence after the muted response to its actions in Georgia.

"If its actions in Ukraine are also forgiven, its next step is an open question," he said.

Mr McCain and Mr Graham asked Mr Trump to take a tough line against Russia over what they termed its "military land grab" in Ukraine.

They have also called for a bipartisan panel to investigate cyber attacks against the US, including Russia's alleged efforts to influence the US presidential election.