US Defence Secretary Robert Gates, on a visit to Afghanistan yesterday, cautioned against over-optimism, warning of "dark days" ahead despite grounds for hope on the battlefield.

Mr Gates, on his first Afghan trip since President Barack Obama's surge of 30,000 troops began arriving in the country last December, said Nato forces had made gains recently, including a push to take control of the Taliban stronghold of Marjah.

"There is still much fighting ahead, and there will assuredly be some dark days. But looking forward there are grounds for optimism," Gates told a news conference with Afghan President Hamid Karzai.

Hours after Mr Gates arrived, militants demonstrated their growing ability to strike inside Afghan cities, with gunmen launching a commando-style raid in the town of Khost near the Pakistani border in the southeast. A Reuters reporter heard a blast and gunfire, and saw smoke rising from the town centre.

Equipped with grenades and an assault rifle, one of the suicide bombers managed to make his way into a disused government building and opened fire on Afghan and US troops in a police compound next door, a local police officer said.

"There were some casualties among the US soldiers which were airlifted by a helicopter," the police officer, Samkeen Ahmad, said close to the site of the attack.

He said four Afghan police were wounded. A spokesman for Nato-led forces said five foreign service members were wounded in two explosions but none killed. He said both suicide bombers had blown themselves up inside the buildings.

The Taliban have increasingly used the tactic of commando-style raids, with bombers and gunmen storming government buildings across southern and eastern towns and in Kabul.

Before his arrival, Mr Gates cautioned against reading too much into "bits and pieces of good news" on both sides of the Afghan-Pakistan border. He said it was too soon to say whether momentum in the eight-year-old conflict had finally shifted.

"I don't think we should lean too far forward in reading too much into specific, positive developments," he told reporters.

"The early signs are encouraging. But I worry that people will get too impatient and think things are better than they actually are. There are still some tough times ahead."

Controlling expectations is critical for Washington and its allies to maintain support for the war amid rising casualties and costs. Mr Obama has said US forces will begin to draw down in July 2011, although officials stress a military role will continue.