NATO summit

"The NATO summit is not a pledging conference, but of course we'll be talking about how our allies can match their resources to the needs identified in the (US) strategic review," Clinton said ahead of her arrival in Strasbourg, France.

Clinton is to accompany US President Barack Obama to the summit, which takes place in Strasbourg and in the neighbouring German cities of Kehl and Baden Baden.

Clinton wants "highest and best" contribution

The US currently provides about half of all the forces taking part in NATO's 62,000-strong International Security Assistance Force (ISAF) mission in Afghanistan. Obama has vowed to deploy an additional 17,000 troops, as well as 4,000 more experts to train Afghan security forces.

Officials in Washington hope that, by setting an example, their European allies will follow.

"What we decided to do was to give the NATO allies, EU members, countries in the region, everyone who we think has a stake in the future of Afghanistan, to do their own internal review and to take a hard look at what they believe is their highest and best contribution," Clinton said.

United States National Security Advisor James Jones said additional help would be particularly needed in the summer, when Afghans go to the polls to vote on whether to renew the mandate of President Hamid Karzai.

"We have a national election coming up. Allies are considering how they might reinforce themselves ... so I expect there will be additional troop contributions," Jones said.

Germany and Italy have already indicated they will be providing more soldiers for the election period, but NATO commanders insist four additional battalions are needed.