Daniel Lak reports: "This is a serious escalation in some of the most intense fighting for years""

The BBC's Delhi correspondent says it is a serious escalation in what was already some of the most intense fighting for years.

Fighter planes and helicopter gunships made six sorties against three targets on the Indian side of the ceasefire line in Kashmir, starting at 6.30am (0100GMT), an Indian official said.

Officials said up to 680 Afghan militants were dug in on the Indian side of the military control line, and a further 400 were waiting to come in.

Owen Bennett-Jones reports: "The Pakistani military have been put on high alert'

India's Defence Ministry said it would continue bombing until what it described as Pakistan-backed forces were driven out.

It also warned Pakistan's air force and army not to interfere.

"Any escalation of this conflict will be entirely the responsibility of Pakistan," a spokesman said.

Pakistan's official news agency said the country's foreign minister, Sartaj Aziz, denied giving logistical backup to the militants.

But he said that Pakistan would take the "necessary action" to defend itself. "We are retaliating and we will retaliate," he said.

Artillery fire

India denied worsening tensions in the region, saying that it had used aircraft because of the inaccessible nature of the terrain. Kargil is mountainous and largely uninhabited.

The Press Trust of India news agency reported that the air force had taken over Srinagar airport and closed it to civilian flights during the operation.

It is the latest escalation of military action in the region, following heavy artillery exchanges between the Indian and Pakistani armies over the last three weeks.

Major General JJ Singh said an Indian army operation to flush out the infiltrators in three areas - Kargil, Dras, and Batalik - could last up to three months.

He said 17 people had been killed, and at least 90 injured on the Indian side.

Indian officials said that nine or 10 infiltrators had been spotted in the region on 6 May. Artillery duels started shortly afterwards and on 10 May India moved approximately 12,000 soldiers into the region.

Another 5,000 were sent in 10 days later, increasing Pakistan's fears that the troops could be there for more than a defensive role.