PESHAWAR, Pakistan (Reuters) - The death toll from a pair of coordinated suicide bombs targeting Shi'ites in a volatile Pakistani town near the Afghan border rose to 52 overnight, officials said on Saturday, in one of the worst sectarian attacks in the country in months.

Tensions between Shi'ite Muslims and Pakistan's majority Sunnis have risen in past years, with radical Sunni Islamist groups frequently attacking Shi'ites whom they see as heretics.

On Friday evening, two suicide bombers on motorcycles struck in the northwestern town of Parachinar just as people flocked to a busy marketplace to buy food for their evening meals after a day of fasting during the holy month of Ramadan.

Riaz Mahsud, the top administrator of the predominantly Shi'ite Kurram area, said the death toll, originally reported at 39, jumped to 52 after several people died from their wounds during the night. He said all the victims were Shi'ites.

A previously unknown group called Ansarul Mujahideen claimed responsibility for the attack.

"We plan more similar attacks against the Shi'ite community in Pakistan to seek revenge for the brutalities of Shi'ites against Sunni Muslims in Syria and Iraq," Abu Baseer, who identified himself as a spokesman for the group, told Reuters by telephone.

Parachinar is home to a significant Shi'ite community which has been previously targeted by militants. Shi'ite Muslims make up a little over 10 percent of Pakistan's population of 180 million.

Separately, gunmen attacked a coast guard checkpoint in the volatile southwestern region of Baluchistan near the Iranian border, firing rockets and killing seven security officials, the region's Home Secretary Akbar Durrani said.

A banned militant group known as the Baloch Liberation Front claimed responsibility for the attack, saying its fighters had killed 25 security personnel and kidnapped two soldiers. Two militants were killed, its spokesman said.

(Reporting by Jibran Ahmad; Writing by Maria Golovnina; Editing by Raissa Kasolowsky)