Intelligence agencies have issued a security alert for Delhi and Uttarakhand following a possible terror threat, with key religious sites, government establishments and police installations put under heightened surveillance, sources said on Saturday.

According to sources in the intelligence establishment, the alert was triggered after an email surfaced warning of possible attacks in the coming days.

This threat comes in the backdrop of a standoff at the Nagrasu gurdwara in Uttarakhand's Rudraprayag district between a group of Nihangs and the authorities. The Nihangs were demanding the release of four of their members arrested in connection with a clash with locals in Karnaprayag market on June 16.

The standoff ended after three days on June 23 following the intervention of a Sikh delegation.

Later, on the night of June 25, a group of Nihang Sikhs forcibly entered Uttarakhand through the Kulhal border in the Dehradun district, which adjoins Himachal Pradesh, demanding the release of the arrested men. Officials managed to persuade them to return to Paonta Sahib.

The four Nihang Sikhs arrested, however, were granted bail on Saturday by a district and sessions court in Uttarakhand.

The email allegedly mentioned several temples in Delhi and Uttarakhand, along with government offices, railway stations and police establishments, as potential targets.

The threat communication also reportedly referred to certain political leaders, raising concerns among security agencies, they said.

Following the alert, Delhi Police, Uttarakhand Police and central intelligence agencies have stepped up vigil and begun reviewing security arrangements at sensitive and high-footfall locations as a precautionary measure.

Officials said deployment has been increased at key religious places, transport hubs and government buildings, while local police units have been directed to remain alert and intensify patrolling.

Sources said Delhi Police has launched a probe into the email to ascertain its authenticity and trace its origin. Cyber experts are examining the digital trail of the message to determine whether the threat is credible or a hoax.

No specific input on the timing or exact location of a possible attack has emerged so far, officials said, adding that preventive measures were being taken in view of the seriousness of the alert.

Security agencies have not ruled out any possibility and are maintaining close surveillance over suspicious activities, the sources added.