SANTIAGO (AFP) - Lawmakers in Chile's lower house of Congress on Thursday approved abortion in limited circumstances, the first step towards lifting a decades-old ban on the practice in the socially conservative South American country.

Under the measure approved by the Chamber of Deputies, abortion will be allowed in cases of rape, risk to the mother, or when the fetus is no longer considered viable.

The Senate must now approve the draft.

Chile is one of the few countries in the world that does not allow abortion under any circumstances at all.

"Incredible - the motion is passed," said the leader of the Chamber of Deputies, Marco Antonio Nunez, after a heated debate of the draft submitted by the government of President Michelle Bachelet.

"This is an historic day. We see the political will to let women make their own decisions," said lawmaker Karol Cariola of the Communist Party, which is part of the ruling coalition.

The deputies voted separately on each of the circumstances under which abortion will be allowed. Each time, the margin of approval was wide.

The measure now goes to the Senate for a vote.

Abortion had been legal in Chile before 1989 in cases of risk to the mother or an unviable fetus.

But ex-dictator General Augusto Pinochet outlawed the practice completely before leaving power in 1990, and the prohibition remained unchanged for more than two decades of democratic rule because of pressure from the Roman Catholic Church and conservative groups.

An estimated 160,000 clandestine abortions are carried out each year in Chile.

Bachelet, who is a pediatrician by training, has taken on conservative groups in Chile by seeking to ease the country's tough abortion law. Polls say 70 per cent of Chileans support the new bill.

Conservative opposition lawmakers decried it as violating the rights of the unborn.

"We could say that the military government, or the dictatorship, whatever you want to call it, killed adults. You kill them before they are born," said lawmaker Manuel Garcia.