(CNN) -- U.S. Defense Secretary Robert Gates plans to press European allies to contribute more to the fight in Afghanistan during two days of informal NATO meetings that began Thursday in the Lithuanian capital.
Gates has made no secret of his frustration with allies reluctant to send more troops and equipment to Afghanistan. Testifying at the Senate Wednesday, before his trip to the meeting in Vilnius, Gates warned that the issue threatens to break the nearly 60-year-old alliance apart.
"I worry a great deal about the alliance evolving into a two-tiered alliance in which you have some allies willing to fight and die to protect people's security and others who are not," Gates said.
Pentagon spokesman Geoff Morrell told reporters en route to Vilnius that Gates wants to be careful not to alienate allies while at the same time encouraging them to do more.
The United States has the most troops in the NATO-led mission, 15,000. Britain has the next-highest number, with 7,800.
Gates has praised the contributions of Britain, Canada, Australia, the Netherlands, and Denmark, telling the Senate those countries "are really out there on the line" and doing the bulk of the fighting. Together with the United States, the countries account for more than half of the 43,250 troops taking part in the mission.
But Gates has said other allies -- specifically Germany, France, Italy, and Turkey -- need to step up their contributions. Specifically, Gates wants NATO countries to send troops to replace 3,200 U.S. Marines being deployed to fight rising Taliban attacks.
"I made specific requests of specific kinds of units and, in some cases, named units and where they needed to go," Gates told the Senate. "I haven't gotten any response as yet."
Gates, who promised to be a "nag" on the issue, said he hopes to get those responses in Vilnius.
Last week, though, Germany declined Gates' request to commit more troops to the Afghan mission and move them to the restive south.
NATO Secretary-General Jaap de Hoop Scheffer hinted at the need for greater contributions as he delivered the meeting's opening remarks.
"We will discuss here in Vilnius how to further strengthen our operation," he said. "But success in Afghanistan means more than just military operations. It requires Afghan ownership, an Afghan face, and a truly comprehensive approach. That means sharing responsibility, doing things together."
Secretary of State Condoleezza Rice pressed the case on a one-day trip to London on Wednesday, where she talked with her counterpart David Miliband about shoring up flagging European support.
The pair traveled to Afghanistan Thursday.
"Our populations need to understand this is not a peacekeeping mission," Rice said in London. "This is a counterinsurgency fight."
British Prime Minister Gordon Brown called for "proper burden-sharing" in Afghanistan, and the top NATO commander there, Gen. Dan McNeill, has called the mission "an under-resourced force."
"The Americans are becoming increasingly frustrated with the inability of the European allies to get their act together," said Michael Williams, an analyst with the Royal United Services Institute, a military think tank. "Not just in terms of burden-sharing, which allies are doing in the north, but risk-sharing."
Vilnius is the first meeting for NATO allies since Gates started urging them to increase their contributions. While Afghanistan will be a major topic, it won't be the only one. Members also plan to discuss the NATO mission in Kosovo and the alliance's relationship with Russia. E-mail to a friend
CNN's Barbara Starr and Robin Oakley contributed to this report.