(Reuters) - Nagorno-Karabakh is a mountainous and heavily-forested patch of land that sits inside the territory of ex-Soviet Azerbaijan.

Under international law, Nagorno-Karabakh is recognized as part of Azerbaijan. But the ethnic Armenians who make up the vast majority of the population reject Azeri rule. They have been running their own affairs, with support from Armenia, since Azerbaijan’s troops were pushed out in a war in the 1990s.

Long-standing ethnic tensions in the region between Christian Armenians and their Muslim neighbors flared up in Nagorno-Karabakh in the late 1980s. At the time, the territory was within the borders of the then-Soviet republic of Azerbaijan, but that made little difference to the Armenian population because most decisions were made in Moscow.

As the Soviet Union started to break-up, it became apparent that Nagorno-Karabakh would come under the direct rule of the government in the Azeri capital, Baku. The ethnic Armenians did not accept that.

Sectarian conflict broke out, escalating into all-out war between Azerbaijan’s troops on one side and, on the other, Nagorno-Karabakh’s ethnic Armenian forces with support from Armenia. Thousands of people were killed and hundreds of thousands were displaced.

By 1994, when an internationally-brokered ceasefire was agreed, the ethnic Armenians were in control of almost all of Nagorno-Karabakh, plus some surrounding districts that provided them with a buffer zone and a land bridge connecting their region to Armenia.

From that point on, the war became one of the former Soviet Union’s “frozen conflicts” - still unresolved but with little actual fighting.

Nagorno-Karabakh’s independence was not widely recognized internationally, leaving the ethnic Armenian administration there in a state of legal limbo and under blockade from Azerbaijan’s government. Azerbaijan vowed to take back control over the territory, even using military force if necessary.

International efforts to find a lasting peace settlement, involving France, the United States and Russia as mediators, came very close on at least one occasion but ultimately failed.

In the meantime, a tense armed standoff developed on the “contact line” that separated the two sides’ forces. There were periodic exchanges of fire, sometimes involving casualties, but the battle lines did not shift.

The fighting that broke out on April 2 was the most intense in years, and prompted warnings that the conflict could spiral once again into all-out war.