NATO member Lithuania said Thursday it trusts US President Donald Trump to make good on his predecessor's commitment to beef up the alliance's eastern flank.

The previous administration of president Barack Obama ordered an unprecedented deployment of troops to reassure NATO allies in eastern Europe after Russia's annexation of the Crimea from Ukraine in 2014.

"We trust the US administration," Lithuanian President Dalia Grybauskaite said at a joint press conference with her Estonian, German and Latvian counterparts in Riga.

"We believe that all obligations will be fulfilled and we will have the same reliable NATO partner and ally as it was before.

"This is done already. We have American troops on our soil," Grybauskaite said. The US deploys rotations of around 120 soldiers in the Baltic state and 10 US tanks are to arrive there on Friday.

Grybauskaite's comments at a time of uncertainty for eastern NATO allies like Poland and the three Baltic states, who have been unsettled by Trump's seemingly pro-Moscow stance coupled with critical remarks about NATO.

Trump has called the alliance "obsolete" in terms of fighting terrorism, but of "fundamental importance" to transatlantic security.

Last summer, NATO ordered continuous troop rotations in four eastern members as a tripwire against Russian adventurism in states formerly under Moscow's control.

Confirming that Thursday's Riga trip would be his last foreign visit as head of state, outgoing German President Joachim Gauck said his choice of destination was "symbolic".

"It sends a conscious political signal that Germany stands at the side of our Baltic partners. The change in the security situation has illustrated the urgency of this," he said.

The Kremlin has denied any territorial ambitions and claims NATO is trying to encircle Russia.

But Moscow's deployment last year of nuclear-capable Iskander missiles into its heavily-militarised Kaliningrad exclave, which borders Lithuania and Poland, and frequent Russian military drills in the region have rattled nearby NATO states.