Sizable shipments of Iranian weapons are being supplied to Taliban militants in Afghanistan, suggesting the government in Tehran is aware of them, U.S. Defense Secretary said Wednesday.

Gates said he has seen new information in the last couple of weeks "that makes it pretty clear there's a fairly substantial flow of weapons" going to militants fighting the coalition of Afghan, U.S. and other foreign troops.

Asked if he believed the government of Iran is involved, Gates said: "I would say I haven't seen any intelligence specifically to this effect, but ... given the quantities that we are seeing it is difficult to believe that it's associated with smuggling or the drug business or that it's taking place without the knowledge of the Iranian government."

The comments contrasted with those of Undersecretary of State Nicholas Burns, who said Wednesday that "there's irrefutable evidence the Iranians are now doing this."

"It's certainly coming from the government of Iran," Burns told CNN. "It's coming from the Iranian Revolutionary Guard corps command, which is a basic unit of the Iranian government."

Gates made his comments during a visit to Ramstein Air Base. He is also traveling to a NATO meeting in Brussels, where he said he will press allies to provide more troops for Afghanistan.

Gates has been frustrated with NATO's commitment in Afghanistan, but said Wednesday that several nations recently had indicated a willingness to increase the size of the force they have their or the length of their stay.

"I think countries are taking this seriously and so I will continue to press in Brussels," he told a press conference at Ramstein.

Senior U.S. officials en route to Germany with Gates on Wednesday laid out the secretary's expectations for the two-day meeting of NATO defense ministers that will begin Thursday. In the nearly six months since the NATO leaders met and promised to fill troop and equipment needs for the Afghan war, there have been only incremental increases.

The U.S. officials said Gates will "make a pitch" for countries to send more trainers for the Afghan National Army and police in an effort to get the Afghan government better able to control its own security.

The officials, who requested anonymity so they could preview the secretary's plans for the session, said coalition forces in Afghanistan still need up to four battalions — or as many as 3,000 combat troops, along with about an equal amount of trainers. Gates has said he would like some NATO and non-NATO nations to contribute some of the training forces.

In addition, NATO allies are also trying to put together training teams that can be embedded with Afghan units. And those also have been slow to come together.

In February and again in April, Gates exhorted NATO allies to bolster their troop commitments in Afghanistan so the alliance could launch its own offensive against the Taliban, and pre-empt what has been an annual spring increase in insurgent attacks.

That offensive was launched, with the aid of additional U.S. troops. And, during a visit to Afghanistan early this month, Gates said the NATO push was making progress. But he also warned that Iranian weapons — which have been responsible for widespread violence and U.S. troop casualties in Iraq — are now increasingly showing up in Afghanistan.

For months, Gates has expressed concern about possible reversals in Afghanistan, which still lacks a self-sustaining military and suffers from the unmet expectations of building an effective central government.

In particular, NATO officials said they have found armor-piercing roadside bombs — known as explosively formed penetrators, or EFPs — in Kabul.

The struggle to pressure NATO countries to live up to their commitments has also prompted Gates to question whether the alliance should continue to mount a 25,000-troop response force.

The U.S. currently has 26,000 troops in Afghanistan, including some 14,000 in the NATO-led force.

Another issue likely to come up during the meeting is the ongoing controversy over the U.S. proposal to site missile defense radars and interceptors in eastern Europe. Russian President Vladimir Putin, in a recent meeting with President Bush offered up an alternative, that would allow joint use of a radar station in Azerbaijan.

Russia has strenuously opposed U.S. plans to put the missile defense systems in Poland and the Czech Republic. Gates is expected to meet with the Russian defense minister.