ADDIS ABABA/NAIROBI (Reuters) - Ethiopia's Prime Minister Abiy Ahmed lauded his troops on Monday for ousting a rebellious northern movement, but the leader of Tigrayan forces said they were still resisting amid fears of a protracted guerrilla conflict.

The nearly month-long war has killed hundreds and probably thousands of people, sent refugees into Sudan, enmeshed Eritrea, and stirred rivalries among Ethiopia's myriad ethnic groups.

Federal forces captured regional capital Mekelle at the weekend and declared victory over the Tigray People's Liberation Front (TPLF), a guerrilla movement-turned-political party that dominated national government for nearly three decades until 2018.

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"Our constitution was attacked but it didn't take us three years, it took us three weeks," Abiy told parliament, comparing his offensive with the American Civil War of the 1860s.

"Our army is disciplined and victorious."

Though the TPLF said Mekelle had been bombarded, Abiy said his troops had not used rockets and had not killed a single civilian in Tigray since starting an offensive in response to an attack on an army base on Nov. 4.

Though the highland city of 500,000 people eventually fell with little resistance, the TPLF said on Sunday it had shot down a plane and retaken one town.

The United States expressed concern about continued fighting and called for a complete end to hostilities and for dialogue. Both Washington and the United Nations urged that human rights be fully respected and that aid groups be allowed access.

Graphic: Map of region - here

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'FIGHTING THE INVADERS'

TPLF leader Debretsion Gebremichael denied reports he had fled to South Sudan and said his forces captured some soldiers from neighbouring Eritrea around Wukro, about 50 km (30 miles) north of Mekelle.

"I'm close to Mekelle in Tigray fighting the invaders," he told Reuters in a text message.

Claims from all sides are difficult to verify as phone and internet links to Tigray have largely been down and access is restricted.

The TPLF has shelled Asmara's airport and accused Eritrea of sending troops to fight with Abiy's forces. Both Eritrea and Ethiopia have denied that accusation.

When he took office in 2018, Abiy pledged to unite Ethiopia's 115 million people, but ethnic bloodshed had killed hundreds and uprooted hundreds of thousands from their homes even before the latest flare-up.

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In Tigray, both sides have spoken of hundreds of fatalities in air strikes and fighting. Diplomats believe the toll is in the thousands.

Abiy's spokeswoman Billene Seyoum dismissed the TPLF's comments that fighting continued, dismissing them as the delusions of a disintegrating criminal clique".

Redwan Hussein, head of a government taskforce on Tigray, told Reuters most casualties occurred at the start of the conflict during a TPLF attack on Ethiopian soldiers, attempts by reinforcements to reach them, and then a mass killing of about 700 civilians.

"Our orders were to leave the cities alone, and to encircle them, to cut the forces that were within the towns from their chain of commands," he added, saying the Republican Guard captured Mekelle without shooting as TPLF conscripts deserted.

The TPLF was not immediately available to comment. On Sunday, the Red Cross said Mekelle's hospitals were low on supplies and bodybags, but did not give casualty figures.

Graphic: Map of refugee movements - here

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GUERRILLA WAR?

It is not clear how many fighters the TPLF has left, but Debretsion's defiance raises the spectre of a drawn-out insurgency. The battle-hardened TPLF helped topple Ethiopia's Marxist dictatorship in 1991 and knows how to exploit its mountains and borders with Sudan and Eritrea.

Abiy, whose parents are from the larger Oromo and Amharic groups, said he had felt Ethiopia's ethnic frictions even when taking office as prime minister, where he felt like a "prisoner." He said security services dominated by Tigrayans discouraged him from travelling round Ethiopia.

There are more than 80 ethnicities in Ethiopia, which operates as a federation of 10 regions run by separate groups.

Abiy said he had directed reforms to reduce Tigrayans in senior military positions from more than 60% to a quarter of the top brass. Tigrayans make up roughly 6% of the population.

Though urging the more than 45,000 refugees in Sudan to return, Abiy said it was suspicious so many of them were young males and if any had a role in an alleged massacre of non-Tigrayans in Mai Kadra they should face justice.

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The government blamed the killings on a Tigrayan youth group with the aid of local forces, but the TPLF denied any collusion.

INTERNATIONAL CONCERN

Despite the government's claim of victory, U.S. Secretary of State Mike Pompeo voiced grave concern about the hostilities. In a telephone call with Abiy, Pompeo called for dialogue and a complete end to the fighting, a State Department spokesman said.

He stressed the importance of protecting civilians, including refugees fleeing into Sudan, and giving international humanitarian organizations access to Tigray, the State Department said.

He urged the government to respect the human rights of Tigrayans and all ethnic groups.

U.N. Secretary-General Antonio Guterres spoke with Abiy on Sunday and called for full respect for human rights and access for humanitarian aid, a U.N. spokesman said.

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"The Secretary General also said that Ethiopia needed a true reconciliation, without discrimination...where every community should feel respected and be part of Ethiopia," he said.

(Reporting by Addis Ababa newsroom, and Duncan Miriri and David Lewis in Nairobi, Arshad Mohammed and Tim Ahmann in Washington, Michelle Nichols at the United Nations, Writing by Andrew Cawthorne; Editing by Angus MacSwan)