US President George W Bush has held talks in Bulgaria on US plans to build a missile defence shield in Europe.
Mr Bush was given an honour guard welcome in Bulgaria
Bulgaria's government, a staunch US ally, is concerned it may be left out of the plan, which would include bases in Poland and the Czech Republic.
The issue has contributed to raised tensions between the US and Russia.
Mr Bush also backed Bulgarian calls to free five nurses sentenced to death in Libya on charges of infecting children with HIV.
Speaking at a news conference with his Bulgarian counterpart Georgy Parvanov in Sofia, Mr Bush said the nurses' release was a "high priority".
Mr Bush spoke as an EU delegation was in Libya attempting to negotiate the release of the five.
"They should be released and they should be allowed to return to their families," he said, adding that the US would make representations to Libya on behalf of the Bulgarian authorities.
Mr Bush's trip to Sofia is the final leg of an eight-day European tour including visits to the G8 summit in Germany, the Czech Republic, Poland, Italy and Albania.
During formal talks with President Parvanov and Bulgarian Prime Minister Sergey Stanishev, Mr Bush thanked Bulgaria for its support in Iraq and Afghanistan.
Thirteen Bulgarians have been killed in Iraq, while the country's 200-strong force in Afghanistan is soon to expand to 800 troops.
More than 3,000 US troops are due to start arriving at a new base in Bulgaria in September, as part of a US policy to move many of its European forces closer to the Middle East.
The former Communist bloc nation joined Nato in 2004, and became a member of the European Union this year.
But Bulgaria remains concerned that despite its loyalty, much of the country would fall outside the range of the US missile shield.
However, talks to include Bulgaria in the missile defence plans will only inflame tensions with nearby Russia, which views the system as a threat and a challenge to its influence in the region, says the BBC's Jonathan Beale.
Russia opposes the plan, which it regards as a threat, and President Vladimir Putin has threatened to point Russian missiles at Europe in response.
The US says its missile shield is not directed at Russia, but at what it considers "rogue states" such as Iran.
On Sunday, Mr Bush received a hero's welcome in Albania, another staunch ally, and reiterated his support for a UN plan for independence for Kosovo.
His sentiments were welcomed in Kosovo but rejected by a spokesman for the Serbian government, which opposes independence.
The G8 failed to reach a consensus on the Kosovo issue at a summit last week.
Russia remains strongly opposed to the blueprint for independence laid out in April by UN special envoy Martti Ahtisaari.