US Defence Secretary Chuck Hagel made a previously unannounced visit to the Afghan capital Kabul on Saturday, the NATO-led coalition confirmed.
Hagel was to visit US troops and meet with coalition officials in Kabul, the military said.
The visit comes amid uncertainty between the US and Afghanistan on a major security deal, which Afghan President Hamid Karzai is refusing to sign.
According to reports, Hagel does not plan to meet the Afghan leader.
Earlier Saturday, Hagel – on a visit to Bahrain – attempted to reassure Gulf allies about continued US commitment to their security, after a recent deal between Iran and major world powers.
“Our success will continue to hinge on America’s military power, and the credibility of our assurances to our allies and partners in the Middle East,” Hagel said.
He added that the United States was not planning to reduce its current presence in the region, saying it had deployed some of its most advanced weapons and intelligence gathering systems to deter any threat.
Under an initial accord reached last month in Geneva, Iran agreed to temporarily curb some of its nuclear activities in return for easing economic sanctions.
The Gulf countries, long suspicious of Iran’s regional agenda, are worried that the deal comes at the expense of their security.
“America’s commitment to this region is proven. And it is enduring,” Hagel told the Manama Dialogue in Bahrain, a regional security forum.
He warned Iran that any failure of the accord could still mean a return to the military option.
“We know diplomacy cannot operate in a vacuum,” Hagel said.
“Iran has been a profoundly destabilizing influence and a nuclear-armed Iran would pose an unacceptable threat to regional and global stability.”
Hagel, however, called on the Gulf states to better integrate their defence systems and offered US assistance to help them set up a unified network of ballistic missile defences.
He also suggested an annual ministerial defence meeting between the Gulf countries and the United Sates. “These meetings can begin in six months.”
Gulf countries have repeatedly accused Iran of stirring up unrest in the region.
Iranian leaders have recently signaled interest in improving ties with their Gulf neighbours.
Earlier this month, Iranian Foreign Minister Mohammad Javad Zarif visited Kuwait, Oman and the United Arab Emirates.