"I am not satisfied that an alliance whose members have over 2 million soldiers, sailors, marines and airmen cannot find the modest additional resources that have been committed for Afghanistan," Gates told a news conference after a meeting of a separate organization of southeast European countries.
The main shortfall is in troops to serve as trainers for the Afghan National Army and the Afghan police.
Gates said he intended to pursue the matter at a NATO defense ministers meeting in the Netherlands this week.
During Monday's meeting here of the Southeast European Defense Ministers, a group that was created in 1996 mainly to promote stability in the Balkans, several countries "indicated that they intend to increase their commitments in Afghanistan and Iraq principally in Afghanistan," Gates told reporters.
He added that those countries did not want to be identified publicly yet because they have not finalized their plans.
Earlier, Slovak officials told Gates that they will send at least 47 more troops to Uruzgan province in southern Afghanistan, where they will work with Dutch forces, Pentagon press secretary Geoff Morrell said. That will increase its troop total in Afghanistan to 125 next year, he said.
Slovakia also will send eight doctors to work at a military hospital in Kabul, the Afghan capital, Morrell said.
In opening remarks to Monday's session, Gates urged members of the Southeast European Defense Ministers to boost their contributions to security efforts in Afghanistan, warning that the group "risks eventual irrelevance" unless it does more to fight terrorism and increase European security cooperation.
In his address, Gates praised the group for sending a small headquarters element to Kabul, Afghanistan, last year and said more such missions should be considered.
"Given the wide range of global threats which confront us, contributions by SEDM members to the war on terrorism are particularly important," Gates said, according to a transcript of his remarks released after the start of the closed-door conference. SEDM is the acronym for the defense organization.
"SEDM risks eventual irrelevance if it is principally only a talk-shop," Gates said. "To sustain and increase SEDM's relevance, member nations must be willing to address these crucial issues."
Gates used Monday meeting to underscore the importance of international assistance for Afghanistan, where violence remains high despite some success this year in blunting a planned Taliban offensive.
Gates has been pushing for more help in Afghanistan from European countries, not only those in the NATO alliance but others with security and other resources that could contribute to stabilizing the country.
After the meeting Gates was headed to the Czech Republic for talks on the U.S. proposal to install a missile-tracking radar there as part of a Europe-based U.S. missile defense system that is strongly opposed by Russia.
Much of the higher levels of violence in Afghanistan has been in the southern and eastern provinces. The insurgents are increasingly using Iraq-style tactics, such as roadside bombs, suicide attacks and kidnappings to hit foreign and Afghan targets around the country.