By Paul Taylor RIGA, Nov 28 (Reuters) - U.S. President George W. Bush, weakened by election setbacks at home and worsening violence in Iraq, meets his NATO allies on Tuesday to seek ways to overcome another major security challenge -- in Afghanistan. As Bush flew into the Baltic state of Estonia on his way to a NATO summit in neighbouring Latvia, his top security adviser denied growing talk that Iraq had plunged into a civil war, but acknowledged that sectarian violence had entered "a new phase". White House national security adviser Stephen Hadley said NATO leaders would affirm their determination to prevail over Islamist militants in Afghanistan, the most remote and complex military mission in the 26-nation alliance's history. "There is a recognition on the part of NATO that this is a terribly important mission, not only from the standpoint of Afghanistan but what it represents in terms of the struggle against al Qaeda and Taliban in the war on terror," Hadley told reporters accompanying Bush to Tallinn. NATO Secretary-General Jaap de Hoop Scheffer said on the eve of the summit he had no concrete offers of more troops to join the 32,000-strong NATO-led peacekeeping force in Afghanistan, and key European nations remain reluctant to lift restrictions on the use of their forces. NATO's military commander says he needs an extra 2,500 soldiers, more helicopters and more flexibility to use existing allied troops in the country. In a stark warning on the eve of the summit, a senior U.S. senator said NATO's future was at stake in the conflict. "If the most prominent alliance in modern history were to fail in its first operation outside of Europe due to a lack of will by its members, the efficacy of NATO and the ability to take joint action against a terrorist threat would be called into question," Republican Richard Lugar told a conference. The warning came as a suicide bomber killed two Canadian soldiers in an attack on an alliance convoy in southern Afghanistan, underlining the risks in NATO's most dangerous ground combat to date. LAME DUCK SUMMIT The U.S.-led invasion of Iraq nearly tore NATO apart in 2003 when France and Germany led opposition to Washington's drive to overthrow Saddam Hussein. Transatlantic relations have healed slowly, with Bush chastened by post-war setbacks in Iraq and the Europeans humbled by the rejection of their own constitution and less tempted to act as a counterweight to Washington in world affairs. But Riga may be too much of a "lame duck" summit to chart a bold new course of cooperation in global security. Bush is constrained by the Democrats' capture of both houses of Congress, while British Prime Minister Tony Blair and French President Jacques Chirac are both almost certainly in their last months in office. Bush telephoned Chirac on the eve of the summit to seek common ground on NATO's mission in Afghanistan. France has refused to allow the alliance's strategic reserve brigade to be sent there or to send its own forces to the lawless south. Chirac wants a diplomatic "contact group" to review and reorganise the alliance's mission in Afghanistan, while Bush will urge European countries to boost troop contributions. "Establishing a Contact Group which brings together the countries of the region, the main countries involved and international organisations, as exists in Kosovo, seems necessary to me," Chirac said in comments due to be published on Tuesday in a number of European newspapers. (Additional reporting by Caren Bohan in Tallinn)
