(This Aug. 21 story has been corrected to say that Guinea changed its transition timeline last year, not last week, in paragraph 12)

ABUJA (Reuters) - West Africa's main regional bloc, ECOWAS, has rejected a proposal by Niger's military junta to hold elections within three years, extending a political impasse that could trigger a military intervention if no agreement is reached following a July coup.

ECOWAS and other international powers have been seeking diplomatic solutions to the July 26 putsch in Niger, the seventh in West and Central Africa in three years.

But after several attempts for dialogue were rejected, the bloc - which has taken a harder stance on Niger than its junta-led neighbours - activated a regional force that military heads have said is ready to deploy if talks fail.

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It doubled down on its threat on Friday, one day before the junta eventually agreed to meet an ECOWAS delegation in the capital Niamey, suggesting new willingness to cooperate.

In a televised address to the nation on Saturday evening, junta leader General Abdourahamane Tiani said coup leaders remained open to dialogue.

But he also said the junta would consult on a transition back to democracy within three years, echoing lengthy timelines proposed by other coup leaders in the region.

ECOWAS Commissioner Abdel-Fatau Musah told Reuters on Monday that the bloc's position remained clear.

"Release Bazoum without preconditions, restore constitutional order without further delay," he said, referring to Niger's ousted president, Mohamed Bazoum. He spoke via WhatsApp in response to a query about the proposed elections delay.

The outcome of "ongoing informal discussions" would determine whether ECOWAS would send another mediation mission to Niger, he added.

The bloc's reputation has been at stake since the recent string of coups eroded democracy in the region, raising doubts over its sway as junta leaders have clung to power. It has butted heads with other military governments requesting several years of preparation to hold elections.

ECOWAS imposed sanctions on Mali last year after interim authorities failed to organise promised polls, lifting them only after a new 2024 deadline was agreed.

Burkina Faso has also agreed to restore civilian rule next year, while Guinea shortened its transition timeline to 24 months last year following pressure by ECOWAS.

Niger has already been hit with a flurry of international sanctions since the coup, including from ECOWAS, piling economic pressure on one of the world's poorest countries.

(Reporting by Camillus Eboh in Abuja; Writing by Sofia Christensen; Editing by Bill Berkrot and Matthew Lewis)