US Defence Secretary Leon Panetta arrived in the Afghan capital on an unannounced trip Thursday, a day after a suicide bombing attack and an alleged errant NATO air strike.
Before his arrival, Panetta told reporters that he wanted to hear an assessment from commanders about a recent rise in insurgent attacks and plans for troop withdrawals.
Panetta said an insurgent assault on Wednesday "was much more organized than we've seen before", with a suicide bomber on a motorcycle and a suicide bomber on foot striking a vehicle park outside a NATO base.
Panetta, who flew to Kabul from New Delhi, said he wanted "to get a sense of just exactly what are the Taliban doing".
"There's been an increase in the attacks," he said, adding that the overall level of violence in the country was still down compared to previous years.
The Pentagon chief said he would confer with the top commander of NATO-led forces, General John Allen, to hear how he planned to maintain security despite Taliban attempts to undermine a gradual transfer of authority from alliance troops to Afghan government forces.
In a brief visit of several hours, he was also due to meet US ambassador to Kabul, Ryan Crocker, before speaking to American troops and holding talks with his Afghan counterpart, Abdul Rahim Wardak, officials said.
Wednesday's attack on a car park crammed with vehicles supplying the largest NATO base in southern Afghanistan left 23 dead and 50 others wounded.
A suicide bomber on a motorcycle struck first and as a crowd gathered to help the victims a second bomber walked into their midst and set off explosives strapped to his body, Kandahar provincial police chief General Abdul Raziq said.
Panetta's trip also came amid fresh allegations of a NATO air strike gone wrong, as the International Security Assistance Force (ISAF) investigated reports that civilians were killed in an air raid on a home in Logar province, south of Kabul.
Local police said 18 civilians, including women and children, were killed in the coalition air raid.
Civilian casualties caused by US and NATO air strikes have been a frequent source of tension between Afghan President Hamid Karzai and the United States, which leads the NATO force in the fight against the Taliban.
Panetta's trip to Kabul comes at the end of a nine-day tour through Asia, including stops in Singapore, Vietnam and India.
During the trip, he portrayed the war in Afghanistan as winding down while the United States shifted its focus to the Asia-Pacific region.
The Asia "rebalance" calls for shifting the majority of the US naval fleet to the Pacific, while forces will rotate through Southeast Asia and elsewhere to help secure vital shipping lanes and counter a more assertive China.
While Afghan security forces were increasingly taking the lead for security, Panetta said the war effort was hampered by the presence of sanctuaries in neighbouring Pakistan that enable Haqqani militants to stage attacks on NATO troops and then escape over the border.
The Haqqani militants "are our enemies and we are going to do whatever we can obviously to confront them when they come across that border", he said.
He said the US would continue to put pressure on Islamabad to go after the Haqqani fighters.
Panetta said the United States was pressing ahead with plans endorsed at a NATO summit last month to hand Afghans the combat lead from mid-2013, with most foreign troops due to depart the unpopular war by the end of 2014.
At the Chicago summit, NATO approved an "irreversible" roadmap to "gradually and responsibly" withdraw 130,000 troops by the end of 2014.