India carried out airstrikes overnight against targets in Pakistan that it said were linked to recent terrorist attacks in Kashmir in which Islamist militants killed 26 people, an escalation that marks the most intense fighting in the hotly contested border region in more than 20 years.

Pakistan claimed to have shot down five of the jets involved in the Indian operation.

The strikes raise the possibility of a broader conflict between the two countries, which both possess nuclear weapons.

Indian strikes targeted nine “terrorist infrastructure” sites across Pakistan’s Punjab province and a portion of the disputed Kashmir territory administered by Pakistan. Pakistan’s government denied that there are terrorist camps in the country and government officials have denied any role in the terror attack that claimed the lives of 26 Hindu tourists at a picnic area in Kashmir last month.

Some of the Indian strikes targeted Punjab, a populous area that India has avoided striking since the last full-scale war between the neighbors more than 50 years ago.

India’s Defense Minister Rajnath Singh said that the targets were “destroyed with exactness according to a well-planned strategy” and that no civilians were killed in the strikes, per Reuters.

The strikes killed 27 people, the authorities in Pakistan said. Prime Minister Shehbaz Sharif called the strikes an “act of war” and promised retaliation.

A Pakistani military spokesman told the news service that the country shot down five of India’s jets. That claim has yet to be independently confirmed.

President Trump called the hostilities “a shame” in comments to reporters.

“I just hope it ends very quickly,” he added.

The Indian operation was named “Sindoor,” the Hindu word for widow, after the widow of an Indian intelligence officer who was killed in the April 22 massacre. India controls Kashmir, though Pakistan also claims the territory, which has made it a long-running flashpoint.