NEW DELHI - Indian Prime Minister Narendra Modi condemned Sunday a "shocking" suicide bomb blast at the main Pakistan-India border crossing that killed at least 55 people.

"Terror attack in Pakistan near Wagah Border is shocking. I strongly condemn such a dastardly act of terrorism," Modi said on his Twitter account.

"My condolences to the families of the deceased. Prayers with the injured."

India has stepped up security along its side of the border in the wake of the attack near the Pakistani city of Lahore that tore through crowds of spectators leaving after a daily ceremony to close the frontier. RPS Jaswal, deputy inspector general of India's Border Security Force which guards Wagah, said security has been increased although "our side is secure". "A red alert has been issued keeping in view the blast across the border in ... Pakistan," Jaswal told AFP by phone from his base near the Indian city of Amritsar.

The neighbours have had frosty relations since independence from Britain in 1947, fighting three full wars, two over the Himalayan territory of Kashmir.

Sunday's attack comes amid heightened tensions between the nuclear-armed countries after a resurgence of cross-border shelling along the disputed frontier in Kashmir.

Ashok Kumar, inspector general of India's Border Security Force, told Reuters the blast had taken place at around 6:15pm local time.

"Our side is safe. We are alert, have increased our security, we are in constant touch with district officials and state police," he said.

Any explosion on the India-Pakistan border is far more serious than a similar event on the Pakistan-Afghan border, another Indian security official said. He said there had not been any major attack in Pakistani Punjab in recent months.