At least four NATO soldiers, two policemen and 10 civilians have been killed in a suicide blast in eastern Afghanistan, police and security sources said. 

Women and children were among the dead and at least 14 people were also injured in Tuesday's attack, which the Taliban claimed responsibility for in a text message to the Reuters news agency.

An Afghan security source told Al Jazeera that the attack took place at a medical clinic in the province of Parwan near the Bagram Air Base.

NATO forces were reportedly investigating a rocket attack on Sunday that hit the base, our correspondent said.

ISAF, the NATO-led international force in the country, released a separate statement confirming that four of its troops were killed. 

The attack came a day after Afghan presidential contender Abdullah Abdullah's camp rejected preliminary results of last month's run-off election as a "coup" against the people, putting him on a dangerous collision course with his rival, Ashraf Ghani.

Al Jazeera's Jennifer Glasse, reporting from Kabul, said the timing of the attack was particularly sensitive. 

"These kinds of attacks are very, very worrying especially at this very politically delicate time with the presidential election results very much in doubt," our correspondent said.   

US Secretary of State John Kerry said on Tuesday that any attempt to take power illegally following a disputed presidential election run-off would cost Afghanistan US support.

"I have noted reports of protests in Afghanistan and of suggestions of a 'parallel government' with the gravest concern," he said in a statement issued by the US embassy in Kabul.

"Any action to take power by extra-legal means will cost Afghanistan the financial and security support of the United States and the international community."

Security has been deteriorating in Afghanistan as most ISAF forces prepare to leave the country after 12 years of war against the Taliban.

Source: Al Jazeera and agencies