KINSHASA, Feb 20 (Reuters) - Thirteen Congolese army soldiers and four former militia fighters have been sentenced to life imprisonment in separate war crimes convictions by local military authorities, the United Nations said on Tuesday.

The sentences were handed down on Monday by military tribunals at Bunia, the capital of Democratic Republic of Congo's northeast Ituri district where government forces, backed by U.N. peacekeepers, have fought for several years to subdue rebels and renegade militias.

Ituri's military court jailed the 13 soldiers, members of the First Integrated Brigade, for life after finding them guilty of the massacre of about 30 civilians whose bodies were found late last year buried in mass graves at Bavi, south of Bunia.

The victims, including women and children, had disappeared during army operations against local militia in late August or early September, during the run-up to Congo's Oct. 29 presidential election run-off.

The elections, won by incumbent President Joseph Kabila, were the first free polls in Congo in more than 40 years and were meant to draw a line under a 1998-2003 war.

The other Ituri life sentences were imposed on four former eastern militia fighters convicted of the 2003 killings of two U.N. military observers, one Jordanian, the other from Malawi.

"These sentences send a strong signal that impunity will not be tolerated," said Kemal Saiki, spokesman for the U.N. peacekeeping mission in Congo.

The men sentenced have five days in which to appeal.

The Congolese soldiers jailed were members of one of several national army units made up of a mix of former government loyalists and rebels who fought during the 1998-2003 war.

This conflict and its resulting humanitarian crisis have killed an estimated 4 million people.

Human rights groups say the fledgling Congolese army is still the country's biggest human rights offender and accuse it of killings, rapes and lootings, particularly during operations to pacify Congo's violent east.

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