WASHINGTON, (AP) – The extra troops that Iraq promised to send into Baghdad in a new U.S.-Iraqi military buildup are arriving on schedule but in inadequate numbers, Defense Secretary Robert Gates said Friday.

Gates was asked at a news conference about Senate testimony on Thursday by the outgoing U.S. commander in Baghdad, Gen. George Casey, who said the arriving Iraqi units have only 55 to 65 percent of their intended troops.

“Fifty-five percent probably isn’t good enough,” Gates said, but he left open the possibility that by the time the Baghdad crackdown begins in earnest the Iraqi combat units will be at full strength.

Gen. Peter Pace, chairman of the Joint Chiefs of Staff, who sat beside Gates in fielding questions at the Pentagon, estimated that the arriving Iraqi units are at about 60 percent of their assigned strength.

“It needs to be stronger than that,” Pace said.

Administration officials have said they expect Iraq to meet the pledges it made, as the troop buildup proceeds, but they have not said explicitly what would happen if the Iraqis fall substantially short on troop contributions.

“Partly it will depend on how quickly they get back up to strength,” Gates said.

The defense secretary has publicly held out the possibility of slowing or stopping the flow of additional U.S. troops if the Iraqis fall short, as they have in the past; the Pentagon has announced plans to send five additional Army brigades, totaling 17,500 troops, to Baghdad by May. In addition, about 4,000 Marines are to be sent to western Anbar province.

At his news conference, Gates also said that the decision announced in January to send a second U.S. aircraft carrier to the Persian Gulf region does not mean the United States is planning for a war with Iran. He said the purpose was to underscore to U.S. allies as well as potential adversaries that the Gulf is a vital interest to the United States.

“Nobody is planning, we are not planning for a war with Iran,” Gates said.

Gates said the United States’ main aim with regard to Iranian influence inside Iraq is to counter what he called networks providing explosives used to make roadside bombs that are powerful enough to destroy a U.S. tank.

“Because we are acting against the Iranians’ activities in Iraq, it has given rise to some of these talks” of U.S. intentions to attack Iran, he said, adding that there is no such plan.

Pace said that over the past month or so, raids against those bomb-supplying networks had netted two Iranians.

Gates said it was too soon to say with confidence whether Iranians were involved in the ambush last week in Karbala, in southern Iraq, that left five American soldiers dead. U.S. officials have said in recent days that they are investigating possible Iranian links.

“The information that I’ve seen is ambiguous,” he said.

Gates also said that U.S. military officers in Baghdad were planning to brief reporters on what is known about Iranian involvement in Iraq but that he and other senior administration officials had intervened to delay the briefing in order to assure that the information to be provided is accurate.

Gates opened his news conference by announcing that he has recommended to President Bush that he nominate Navy Adm. Timothy Keating, currently the commander of U.S. Northern Command, to be the next commander of U.S. Pacific Command, replacing Adm. William Fallon, who has been selected as the next commander of U.S. Central Command.

Gates said he also recommended that his senior military aide, Lt. Gen. Victor “Gene” Renuart, be nominated to replace Keating at Northern Command.