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MANILA, April 18 (Reuters) - Two men on a motorcycle were killed in the southern Philippine city of Cotabato on Wednesday when the bomb they were believed to be carrying went off, police said.

"It appeared that the bomb could have exploded prematurely," said police spokesman Willy Casuyo. "Because of the impact of the explosion, the bodies of two people were cut into parts and the vehicle that carried them was also torn into pieces."

Casuyo said they were investigating whether the men were planning a politically-motivated attack.

Violence is mounting ahead of congressional elections on May 14 and the south of the country, where Muslim and communist insurgents are waging long-running rebellions, is a traditional hotspot for poll-related violence.

A pro-administration group held a rally in Cotabato city, around 550 miles (880 km) south of Manila, earlier on Wednesday.

At least 30 people, including political candidates and campaign workers, have been killed in the past two months. Competition for lucrative public office seats and rivalry among political clans fuels the violence.

Tensions have also been rising on Jolo island, west of Cotabato, where 22 people, including two civilians, have been killed and thousands evacuated due to fighting between government troops and Muslim rebels.

The military is hunting Moro National Liberation Front (MNLF) commander Habier Malik after he fired mortars at a marine base on Friday night in retaliation for the army killing two MNLF rebels.

Analysts say the armed forces, who bombarded Malik's camp with 250 pound bombs, have responded with a major show of force because Malik embarrassed them by taking a top general hostage in February.

The general was released unharmed after the government handed over money and sacks of food.

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