The BBC's Subir Bhaumik on the border of the Indian state of Manipur and Burma

At least one policeman and two election officials have been killed in a rebel ambush ahead of Thursday's state elections in India's north-eastern Manipur state.

The first phase of voting was held last week

Authorities said another group of policemen sent to rescue their colleagues also came under fire.

India has deployed thousands of border guards and federal paramilitary troops along the hilly border that separates Manipur and Burma.

Separatist rebels have threatened large-scale violence during Thursday's second phase of voting for the Manipur state assembly.

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State police chief AA Siddiqui said he was expecting trouble, but added he was pleased the rebels were at least participating in the electoral process by backing candidates.

Heavy fire

Thousands have died in more than two decades of separatist militancy in Manipur fuelled by overlapping and competing claims for various ethnic homelands.

I would say that it is a positive development to the extent that you see undergrounds are participating in a way in the election process

A A Siddiqui Manipur police chief

It is not yet clear which of the state's many militant groups was responsible for the latest attack, which occurred on Wednesday in the state's northern district of Senapati.

Police officials said a convoy of three vehicles carrying election officials came under heavy fire from both sides of a hilly road.

One policemen and two election officials were killed on the spot, and five policemen were injured, some seriously, before the militants fled.

Police officials say a group of police commandoes who were sent to rescue the beleagured convoy also came under heavy fire at a nearby hilltop village.

Fighting is said to be continuing.

Remote districts

This is the first time Indian security forces have moved into remote border areas in Manipur, where rebel separatist groups have long been active.

Manipur has been ravaged by two decades of violence

Last week's first round of voting in 40 constituencies was marred by a rebel ambush in which two border guards on election-duty were killed.

The state's remaining 20 constituencies, all in hill districts, go to the polls on Thursday.

Guerrillas of the National Socialist Council of Nagaland are reportedly trying to ensure the victory of candidates seen to be favourably disposed to their political goals.

Militants belonging to the majority Meitei plains community are reportedly trying to ensure their defeat.

The Meiteis want no compromise with the territorial boundaries of Manipur but the Nagas want Naga-dominated areas of the state to be attached with the neighbouring state of Nagaland to create a Greater Naga state.

Positive developments

But police chief Siddiqui said he was heartened that rebels were at least taking an interest in the election by backing different candidates more openly.

He said dialogue had begun with most of the separatist groups active in the state and he hoped the process would continue after the elections.

"I am sure the government of India would be definitely very keen to bring them back into the mainstream," he said.

But he said violence between "rival outfits" remained a real threat, especially in the most inaccessible areas where troop deployment was the most difficult.

According to the AFP news agency, militant groups have attacked the homes of about a dozen former ministers in the past two weeks and killed at least nine people.

The majority of the 368 candidates contesting the vote in Manipur have also received extortion notices from separatist groups.

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