LONDON -- Thousands of ethnic Armenian refugees have started fleeing from the disputed enclave of Nagorno-Karabakh, amid growing fears of an exodus following Azerbaijan’s successful military offensive to retake control of the region last week.

Nearly 3,000 people have already crossed the border into Armenia as of Monday morning, according to an Armenian government statement quoted by the Russian state news agency TASS.

An advisor to the enclave’s ethnic Armenians leadership on Sunday told Reuters that virtually its entire population -- estimated at 120,000 -- would now leave. If they stayed, they would be “ethnically cleansed” by Azerbaijan, he said.

Reporters on the border reported dozens of civilian cars and other vehicles have been driving to the crossing. Reuters reported that groups of civilians in the region’s capital, called Stepanakert by Armenians, were seen loading and packing belongings onto buses.

Azerbaijan blockaded the region for nine months prior to its offensive and controls the only main route out. On Sunday it permitted the first civilians to leave, reportedly escorted by Russian peacekeepers.

Azerbaijan launched a lightening offensive last week that defeated the ethnic Armenia authorities in the enclave within two days of fighting, prompting them to lay down their arms and agree to disband their military forces. Nagorno-Karabakh is recognized internationally as part of Azerbaijan but has been controlled by ethnic Armenians for most of the last 35 years since a war amid the break up of the Soviet Union.

Hundreds of thousands of Azerbaijani civilians were also driven out of Karabakh by Armenian forces during the war in the 1990s when the Armenians were able to establish control.

Ethnic Armenians in the enclave have said they are unwilling to remain there under Azerbaijani rule, saying they fear persecution.

Western countries, including France, Germant and the United States, have expressed fears for the security of the Armenian population.

Armenian authorities said they are prepared for tens of thousands of families to flee.

Azerbaijani troops have been halted on the edge of the region’s capital since end of the offensive, which saw Azerbaijan already seize a number of villages.

Azerbaijan has said it wants to “reintegrate” the Armenian population but has not presented any plan for doing so or for safeguarding their rights. In areas of Nagorno-Karabakh that it has previously retaken, Azerbaijan has encouraged Azerbaijanis to come resettle.