The US commander in Afghanistan has warned troops that they face an increased threat of attack after a series of inflammatory anti-US comments by President Hamid Karzai.

NATO's International Assistance Security Force (ISAF) on Thursday confirmed the contents of a strongly-worded advisory sent by US General Joseph Dunford to his senior commanders on Wednesday.

Karzai on Sunday accused the United States of colluding with militants to justify its presence in Afghanistan and banned international troops from university campuses due to unproven claims of harassment of students.

"Karzai's remarks could be a catalyst for some to lash out against our forces -- he may also issue orders that put our forces at risk," Dunford said in the advisory, which was obtained by The New York Times.

The president has also clashed with the US military over repeated delays to the scheduled handover of Afghan detainees.

The allegation of collusion provoked fury among US officials, as both nations negotiate the framework that could allow some American troops to remain in Afghanistan when NATO combat troops leave next year.

"This advisory was prudent given increased coalition casualties in recent days. General Dunford's email is simply an example of this vigilance," ISAF said in a statement.

Seven American soldiers died on Monday, the deadliest day for NATO troops in Afghanistan so far this year.

Two US soldiers were killed and 10 wounded in a suspected "insider" attack by a man dressed in Afghan army uniform. Five Americans were also killed in a helicopter crash, blamed on bad weather.

Last month Karzai ordered US special forces out of Wardak, a strategic province adjacent to Kabul, and stopped Afghan forces from calling in US air strikes.

"We're at a rough point in the relationship," Dunford said in his advisory. "(Militants) are also watching and will look for a way to exploit the situation -- they have already ramped up for the spring."

NATO is training Afghan soldiers and police to take over the fight against the Taliban as 100,000 international troops prepare to head home by the end of 2014.

His recent outbursts have also triggered criticism from rival politicians, who say he is deliberately misleading ordinary Afghans and could threaten the future of international aid on which the country relies.

"He says the Taliban and Americans are one, and that Taliban came here for strengthening the Americans," Abdullah Abdullah, a former presidential candidate, said in a speech on Thursday.

"Our people are worried about whether Karzai is right -- or they ask whether it is another conspiracy. I was not surprised the remarks, but I was saddened."

The president's anti-US rhetoric has been widely seen as an attempt to establish himself as a strong nationalist leader before he steps down at the end of his second term next year, 13 years after the Taliban were ousted.