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Defense Secretary Robert Gates said Tuesday that sending a second U.S. aircraft carrier to the Persian Gulf could serve as a "reminder" to Iran, but he said it's not an escalation of force.

Speaking to reporters after meeting with Mexican leaders, Gates said heightening U.S. criticism of Iran and its support for terror groups is not a signal that the administration is laying the groundwork for a strike against Tehran.

Still, he said Iran continues to back the Taliban in Afghanistan.

"I do not have a sense at this point of a significant increase in Iranian support for the Taliban and others opposing the government in Afghanistan," Gates said. "There is, as best I can tell, a continuing flow, but I would still characterize it as relatively modest."

His comments contrasted with those from Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, who said last week that he had not seen any new signs of Iranian support for the Taliban.

Gates played down the addition of a second carrier to the Gulf, saying that the number of ships there rises and falls continuously. He said he doesn't expect there to two carriers there for a long time.

Asked if the carrier move went hand in hand with the rising U.S. rhetoric against Iran, Gates said, "I don't see it as an escalation. I think it could be seen, though, as a reminder."

In the past, military officials have said that beefing up the Navy's presence in the Gulf was a way to show that that the U.S. remains committed to the region. And they have acknowledged it also serves as a show of force for other countries there, such as Iran.

In recent weeks, U.S. officials have ratcheted up their complaints that Iran is increasing its efforts to supply weapons and training to militants in Iraq.

Military commanders in Baghdad are expected to roll out evidence of that support soon - including date stamps on newly found weapons caches showing that recently made Iranian weapons are flowing into Iraq at a steadily increasing rate.

Another senior military official said the evidence will include mortars, rockets, small arms, roadside bombs and armor-piercing explosives - known as explosively formed penetrators or EFPs - that troops have discovered in caches in recent months. The official, who spoke on condition of anonymity because the evidence has not yet been made public, said that dates on some of the weapons were well after Tehran signaled late last year that it was scaling back aid to insurgents.

Speaking of Afghanistan on Tuesday, Gates said that the Taliban is changing its tactics there - from large-scale firefighters to a "significant increase in terrorist acts," including roadside bombs and suicide attacks, similar to the one that unsuccessfully targeted Afghan President Hamid Karzai on Sunday.

At least one police officer has been arrested in connection with the assassination attempt, deepening concerns about the Taliban's infiltration of Afghan security forces.

Gates said, however, he does not have a sense that the infiltration is any worse that it has been before. He said that it is important to screen the security forces well and that military trainers working with those forces need to make that a focus of their efforts.

On other matters, Gates said it was critically important for Congress to approve funding for an anti-drug trafficking program to aid Mexico.

The so-called Merida Initiative is a $500 million proposal to counter drug crime in Mexico. While the Pentagon portion of the program is small, the U.S. military has worked with Mexico to provide intelligence, surveillance and equipment to counter drug cartels.

"We have a shared concern and a shared threat in the drug cartels," Gates said, adding that it will benefit the U.S. to enhance Mexico's ability to deal with them.

He added that if Congress failed to approve the funding - which is included in the emergency war funding bill - it would be "a real slap at Mexico."

This is Gates first trip to Mexico City and only the second visit in recent history by a Pentagon chief. The only other U.S. defense leader to travel to Mexico was William Perry, in 1985.