KABUL: US Defense Secretary Robert Gates held talks in Kabul on Monday, warning of "hard fighting" ahead as his ground commander said NATO could take on the Taliban in their spiritual capital this summer.

"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 in order to review US-led efforts to beat back the Taliban.

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 under a last-ditch 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 neighbouring 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 pledged by President Barack Obama in December have arrived in Afghanistan, 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 discussed the offensive -- billed as the biggest since the 2001 US-led invasion -- with Afghan President Hamid Karzai and the commander of US and NATO troops, General Stanley McChrystal, as well as operations planned this year.

McChrystal said NATO and US troops could take on the Taliban in their Kandahar strongholds this summer when enough reinforcements are on the ground.

"We are absolutely going to secure Kandahar," he told reporters, saying there would be significantly increased troop levels by "early summer," but stopped short of announcing a timeline for an offensive.

"There won't be a D-Day that is climactic, it will be a rising tide of security as it comes," he said.

Although Kandahar was not under direct Taliban control, it was "under a menacing Taliban presence particularly in the districts around it," he added.

Gates's visit came as the foreign minister of Washington's arch-foe, Iran, accused US and British forces of fomenting terrorism in the region.

Iran's Mehr news agency initially reported that President Mahmoud Ahmadinejad was scheduled to arrive for talks with Karzai on Monday, but an official in his office said he would not in fact visit.

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 the US-led invasion brought down in 2001, but US officials have long accused Iran of maintaining links to Islamist insurgents in Afghanistan.

"They also understand our reaction -- should they get too aggressive in this -- is not one they would want to think about," Gates said.

Such a US response would be carried out "within Afghanistan", Gates's press secretary said later.

Gates said he would discuss with Karzai the president's plans to promote reconciliation with the Taliban and other insurgent leaders.

But he denied there was any "serious gap" in opinion among NATO allies on the issue, saying all agreed any bid to persuade the Taliban to choose politics over violence had to be "Afghan-led".

He repeated his view, however, that senior Taliban leaders would only be ready to 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."

Gates also said he would raise allegations that the Afghan administrator for Marjah, Abdul Zahir, had been jailed in Germany for assaulting his step-son.

"The question is if the guy committed a crime and served the time, does that automatically rule him out? I mean I just don't know the answer to the question," Gates said.