US Defense Secretary Robert Gates flew into Kabul on Monday on an unannounced visit, warning of "hard fighting" still ahead despite signs of progress in the eight-year war against Taliban insurgents.
"There is no doubt there are positive developments going on, but I would say it's very early yet," Gates told reporters on his plane before landing in the Afghan capital.
He cautioned that there would be "some very hard fighting, very hard days ahead" as US, NATO and Afghan forces step up pressure on Taliban militants in the south as part of a new strategy designed to end the war.
Gates acknowledged "bits and pieces of good news" when asked about the recent capture of senior Taliban leaders in neighboring Pakistan, but said it was probably too soon to say momentum had shifted to coalition forces.
"I think more needs to be done," he said, adding that a surge of US reinforcements was still in its initial stages.
About 6,000 of the 30,000 additional troops have arrived in Afghanistan since President Barack Obama announced the surge in December, Gates said, with the rest due to deploy by the end of August.
It was the Pentagon chief's first visit to Afghanistan since NATO and Afghan troops swept into the former Taliban stronghold of Marjah on February 13, in an assault seen as a pivotal test of Obama's bid to turn around the war.
Gates said he would discuss the results of the offensive -- billed as the biggest since the 2001 US-led invasion -- with the commander of US and NATO troops, General Stanley McChrystal, as well as operations planned this year.
Military leaders have said coalition forces will move on to other Taliban bastions in the south, and have signalled that Kandahar -- the militia's spiritual capital which neighbours Helmand -- will likely be the next target.
Gates said he wanted to "get an update on the campaign not only in Marjah but the next steps as we look to spring, summer and fall".
The defense secretary was also due Monday to meet Afghan President Hamid Karzai, who visited Marjah on Sunday and appealed to wary residents to back his government.
Gates's visit came as the president of Washington's arch-foe, Iran, was scheduled to arrive for talks with Karzai, Iran's Mehr news agency reported, amid rising tensions over Tehran's nuclear program. Iranian President Mahmoud Ahmadinejad has repeatedly called for the withdrawal of US-led troops from Afghanistan, which has close ethnic and religious ties to Shiite Iran.
Tehran bitterly opposed the Taliban regime that ruled in Kabul from 1996 to 2001, but US officials have long accused Iran of maintaining links to Islamist insurgents in Afghanistan.
"They want the Afghan government to be friendly to them but they do not want us to be successful," Gates said.
Iran appeared to provide some "low-level support" to militants and "could do a lot more" if it chose to, he said.
"They also understand our reaction, should they get too aggressive in this, is not one they would want to think about," he added.
Such a US response would be carried out "within Afghanistan", Gates's press secretary said later. In his talks with Karzai, Gates said he would discuss the president's plans to promote reconciliation with Taliban and other insurgent leaders at peace talks later this year.
"I think frankly we need to flesh out these ideas and see what President Karzai has in mind," the Pentagon chief said.
He denied there was any "serious gap" in opinion among NATO allies on the issue, and that all agreed any bid to persuade the Taliban to choose politics over violence had to be "Afghan-led".
But he repeated his view that senior Taliban leaders would only be ready to make concessions and lay down their arms "when they see the likelihood of their being successful has been cast into serious doubt".
"My guess is they're not at that point yet."