Nato Secretary General Jaap de Hoop Scheffer has asked the 26 members of the alliance to heed calls for more international troops for Afghanistan.
Nato troops make up much of the force in Afghanistan
He told the BBC some Nato members were carrying more of the burden than others in the fight against the Taleban.
Nato leaders have asked members to pledge some 2,500 extra troops.
Nato leads the International Security and Assistance Force (Isaf) in Afghanistan, which currently includes at least 18,500 personnel.
Some 37 nations have committed troops to the Isaf deployment.
Conflict between international troops and Taleban fighters has grown in intensity this year, especially in the southern regions of Kandahar and Helmand.
The militants, who were ousted from government in 2001 by Nato-backed forces, have proved to be a far more determined enemy than anticipated, says the BBC's Alastair Leithead in the capital, Kabul.
Speaking to the BBC a day after meeting alliance ambassadors, Mr de Hoop Scheffer said he was working continuously in an effort to boost Nato numbers in Afghanistan.
"The question here is that nations should live up to what they promised," he told BBC Radio 4's Today programme.
"A 26-nation alliance should show this solidarity."
Nato officials are to hold a "force generation conference" on the troop appeal in Belgium on Wednesday.
The secretary general's call was backed by the international commander in Afghanistan, Lt Gen David Richards.
Gen Richards told the Financial Times that Nato members knew 18 months ago that commanders in Afghanistan wanted a reserve force of 1,000 troops, plus 1,500 air support troops which the alliance's supreme commander called for last week.
"That requirement has never been met by nations. The bit it lacked was a hard-hitting reserve of about 1,000 people that I can use wherever I need to use it throughout Afghanistan, although obviously its focus would be the south."
Despite taking regular casualties, Gen Richards insisted international forces were establishing "psychological ascendancy" over the Taleban.
Nevertheless, some commanders apparently remain concerned about the dangers of committing troops to southern Afghanistan.
In some cases, our correspondent says, they are unwilling even to change rules of engagement to allow the movement of soldiers from quieter and safer parts of the country to fight in the south.
There is a serious concern on the ground that the extra troops just will not be found and Isaf will have to make do with what it has got, under-resourced or not, he adds.
Meanwhile, the head of the United Nations Office on Drugs and Crime, Antonio Maria Costa, has called on Nato to take military action against opium production in southern Afghanistan.
He said Nato forces should destroy heroin laboratories, disband public opium markets, attack opium convoys and bring major traders to justice.
Mr Costa told the BBC, however, that in order to eradicate drugs cultivation in Afghanistan, the government also needed to assist opium farmers economically.