"Our reports say militants belonging to the United Liberation Front of Asom (ULFA) are still in their camps inside Bhutan," Khagen Sharma, head of police intelligence in the northeastern Indian state of Assam, told AFP.

The ULFA is fighting for an independent homeland in Assam.

Indian intelligence officials say there could be more than 3,000 ULFA rebels in Bhutan who carry out hit-and-run guerrilla strikes on soldiers in Assam.

In March Bhutan issued an ultimatum to ULFA to vacate its camps by June 30, failing which it said the government would remove them by military might.

"If the militants refuse to leave our country through the process of peaceful dialogue, we will be left with no option but to use our military forces to remove them," Bhutan's King Jigme Singhye Wangchuk said last month while announcing the deadline.

Bhutan issued the ultimatum after New Delhi sent its National Security Adviser, Brajesh Mishra, to the capital Thimphu to air India's serious concern at the presence of the Assamese militants in Bhutanese territory.

Bhutan says ULFA has nine camps in the country, including the outfit's general headquarters and military training bases inside thick jungles in the southern Samdrup Jhonkar district.