The US commander in Afghanistan has warned his troops face an increased threat of attack from militants and rogue Afghan forces after a series of inflammatory anti-US comments from President Hamid Karzai.

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

"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 obtained by The New York Times.

Dunford's warning came after Karzai accused the US of colluding with the Taliban to justify its presence in Afghanistan and banned foreign troops from university campuses due to unproven claims of harassment of students.

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 Thursday.

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, while five Americans were killed in a helicopter crash that was 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."

Speaking Sunday after two suicide bombers killed 19 people, Karzai accused the United States of colluding with the Taliban and said such attacks enabled the US-led military force to justify its presence in Afghanistan.

After an outraged response, Karzai Thursday said his comments were intended to "correct" rather than damage US-Afghan relations and acknowledged there were serious bilateral strains as the NATO-led coalition starts to withdraw.

"The president called the United States a friend and strategic partner of Afghanistan and said his recent comments... had been to correct rather than damage this relationship," a statement from his office said.

"The president said that both countries are in a critical stage of relations, therefore it is natural that each side tries to stand for and focus on its national interests."

Karzai told a debate forum held at his palace in Kabul that among the causes of tension were a dispute over Afghan detainees held by the US and civilian casualties in NATO military operations.

"We want a good relationship with America, we want friendship, but friendship between two sovereign nations," Karzai told the audience of invited guests and government officials.

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.