JOHANNESBURG (Reuters) - Zimbabwe's army is supplying militants with weapons to intimidate voters to ensure that Robert Mugabe wins a possible runoff in the presidential election, Human Rights Watch said.

In a statement released late on Tuesday, it said military forces had equipped war veterans with weapons and trucks to scare Zimbabweans into backing Mugabe.

"The army and its allies - 'war-veterans' and supporters of the ruling party ZANU-PF - are intensifying their brutal grip on wide swathes of rural Zimbabwe to ensure that a possible second round of presidential elections goes their way," said Georgette Gagnon, Africa director at Human Rights Watch.

The group called on the African Union and the United Nations Security Council to take immediate steps to prevent an escalation of violence.

Morgan Tsvangirai's Movement for Democratic Change (MDC) has accused Mugabe of unleashing militias to help him rig victory and activating veterans of the independence war, who have used violence in the past to support the former guerrilla leader.

The MDC has said at least 15 party members have been killed by ZANU-PF. The government denies accusations that it has launched a violent campaign against opponents in the post-election crisis.

"According to scores of victims and eyewitnesses interviewed by Human Rights Watch, ZANU-PF supporters and "war veterans" are drawing up lists of MDC activists who are then systematically targeted for abuse," said the Human Rights Watch statement.

"These ZANU-PF allies are also forcing people to attend meetings to swear allegiance to ZANU-PF and denounce those remaining MDC supporters."

In one example of the violence, Human Rights Watch said, a group of war veterans and ZANU-PF supporters fired at a group of 22 MDC activists who had inquired about the whereabouts of 12 MDC supporters on April 23, in Manicaland.

The U.N. Security Council took up Zimbabwe's election standoff for the first time on Tuesday, and Western powers pressed for a U.N. mission or envoy to visit the crisis-stricken southern African country.

Vote checking from Zimbabwe's March 29 presidential poll was put off again on Tuesday, further delaying when the world will know if Mugabe will remain in power.