Ethiopian troops and their allies are waging air and ground strikes against Tigray rebels in the northern region of Amhara, humanitarian and rebel sources told AFP.
The bombardments hit several areas of Amhara on Thursday and Friday, the humanitarian sources said, amid growing speculation of a major push by government forces against the rebels.
There is a "massive move" against the rebels, said Getachew Reda, spokesman for the Tigray People's Liberation Front (TPLF), which has been locked in a brutal conflict with pro-government forces in northern Ethiopia for almost 11 months.
The reports come just days after Prime Minister Abiy Ahmed was sworn in for a new term on Monday, vowing to stand strong and defend "Ethiopia's honour" despite mounting international criticism of the conflict and alarm about the humanitarian crisis it has triggered.
Getachew said there had been "mostly air, drone and artillery bombardment" of TPLF rebels, and reported a major troop build-up, saying "tens of thousands are amassed" in northern parts of Amhara including the North Gondar and North Wollo zones.
"We are confident we will thwart the offensive in all fronts and more," he said. "We will stand our ground until the siege is lifted."
Representatives from Amhara, as well as federal and military officials, did not respond to requests for comment about the reported military operations, which could not be independently confirmed by AFP.
The war erupted in November when Abiy sent troops into Tigray to topple the TPLF, the regional ruling party and former national rulers, a move he said came in response to TPLF attacks on federal army camps.
Although government forces quickly took control of Tigray's cities and towns, the TPLF recaptured most of the region including the capital Mekele by late June.
Fighting has since spread to the neighbouring Amhara and Afar regions, and caused what the United Nations has called an "immense humanitarian crisis" with hundreds of thousands of people driven into famine-like conditions.
An untold number of civilians have been killed, nearly two million have been displaced, and there have been many reports of atrocities including massacres and mass rape.
- Mass mobilisation -
There has been speculation that the fighting could pick up now that the rainy season is ending, and with mass mobilisation throughout the country and in Amhara in particular.
On Thursday, the spokesman for the Amhara region said on Twitter: "Since an operation to free our people who are in trouble because of the terrorist TPLF could be conducted at any time, in all fronts, we all have to be vigilant 24 hours a day."
Awet Weldemichael, a Horn of Africa security expert at Queen's University in Canada, said earlier this week that Abiy's new government would have to grapple with what he called "a trifecta of crises" -- the war itself and its humanitarian and economic fallout.
"The upcoming wave of fighting and worsening humanitarian conditions are a further hit on his international standing and a test to his new government from day one," he told AFP.
Abiy's government sparked global outrage last week when it expelled seven senior UN officials from Ethiopia for "meddling" in its affairs, exacerbating concerns about the humanitarian response in Tigray.
UN chief Antonio Guterres, who has said more than five million people were in need of humanitarian assistance, on Wednesday urged the Ethiopian authorities to allow the UN to deliver aid "without hindrance".
Tigray is under a de facto blockade that is preventing most aid getting in. Ethiopian officials blame the TPLF for obstructing deliveries, but the US State Department told AFP last month that access to essential supplies and services was being denied by the Ethiopian government.
The UN's humanitarian agency OCHA has in recent days warned that a lack of medical supplies was also having fatal consequences in Tigray and reported alarming levels of malnutrition among children and pregnant women.