At least 45 people were killed on Sunday when a suicide bomber blew himself up outside a restaurant on the Pakistani-Indian border, just after a daily ceremony when troops from both sides simultaneously lower the two nations' flags, officials said.

Hundreds of people visit the Wahga border crossing near the Pakistani city of Lahore to witness the flags of both countries being lowered just before sunset.

"I was sitting in my office near the border when I heard the blast. I rushed to the scene and saw scattered bodies, injured men, women and children and smashed cars," a Pakistani intelligence source told Reuters.

Media reports earlier said the explosion was caused by a gas cylinder but later police said it was probably a suicide bomber.

Police said they were investigating, and a doctor said up to 70 people were wounded.

"According to initial information it was a suicide attack," Inspector General of Punjab Police, Mushtaq Sukhera, told local television channels.

"When ... security was a bit relaxed, the suicide attacker blew himself up near a restaurant. Thirty-seven people were killed and more than 50 wounded," he added.

The Indian side of the main border crossing was not affected by the bombing.

Ashok Kumar, inspector general of India's Border Security Force, said the blast took place 500 meters from the Wagah border at about 6:15 PM local time.

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