ADDIS ABABA (Reuters) - At least 100 people, including students, were kidnapped for ransom last week in Ethiopia's restive regions that have seen sporadic fighting since the end of the civil war in Tigray, the U.S. ambassador to Addis Ababa said on Monday.

While a peace agreement signed in November 2022 has resulted in a measure of stability for Tigray, Prime Minister Abiy Ahmed's government has struggled to bring security to other areas.

Last year, more than 1,300 people were killed across the country, with most of the violence affecting the regions of Amhara and Oromia, according to the United Nations.

"Recent and frequent kidnappings in Oromia and Amhara regions show how prolonged conflict emboldens criminals and weakens (the) rule of law," U.S. Ambassador Ervin Massinga wrote on social media platform X.

"Last week, over 100 students and passengers were abducted for ransom," he added.

On Wednesday, three buses were stopped by unknown gunmen around 120 km (75 miles) north of the capital Addis Ababa, in the Oromia region, said a student of Debark University, who later escaped and hid in a forest.

"It was scary and shocking. They started to beat the passengers with sticks, and force them to get out of the bus," he said.

The attackers spoke Oromo, he said, and had the same hairstyle as fighters from the Oromo Liberation Army (OLA) rebel group, which the UN has accused of killings, destruction of property, rape and abductions.

"The kidnappers are now asking families to pay up to 1 million birr ($17,500) to release the captives," he told Reuters, requesting anonymity.

Spokespersons for the OLA, local Oromia administration and central government did not immediately respond to Reuters' requests for comment.

Asmamaw Zegeye, Debark University's president, confirmed the incident but did not provide further details.

The sister of another student said the group had asked for 500,000 birr for their release.

($1 = 57.3752 birr)

(Reporting by Dawit Endeshaw; Writing by Hereward Holland and Rod Nickel)