TBILISI/BAKU (Reuters) – Armenian and Azerbaijani officials said on Thursday that they had agreed on the text of a peace agreement to end nearly four decades of conflict between the South Caucasus countries, a sudden breakthrough in a fitful and often bitter peace process.

The two post-Soviet countries have fought a series of wars since the late 1980s when Nagorno-Karabakh, a region in Azerbaijan that had a mostly ethnic-Armenian population at the time, broke away from Azerbaijan with support from Armenia.

Armenia’s Foreign Ministry said in a statement on Thursday that a draft peace agreement with Azerbaijan had been finalized from its side.

“The peace agreement is ready for signing. The Republic of Armenia is ready to start consultations with the Republic of Azerbaijan on the date and place of signing the agreement.”

In its statement, Azerbaijan’s Foreign Ministry said: “We note with satisfaction that the negotiations on the text of the draft Agreement on Peace and the Establishment of Interstate Relations between Azerbaijan and Armenia have been concluded.”

However, the timeline for signing the deal is uncertain as Azerbaijan has said a prerequisite for its signature is a change to Armenia’s constitution, which it says makes implicit claims to its territory.

Armenia denies such claims, but Prime Minister Nikol Pashinyan has said repeatedly in recent months that the country’s founding document needs to be replaced and has called for a referendum to do so. No date has been set.

Russia’s TASS state news agency cited Mr. Pashinyan as telling journalists on Thursday that the agreement would prevent personnel from third countries deploying along the Armenia-Azerbaijan border.

That provision would likely cover a European Union civilian monitoring mission that Baku has criticized, as well as Russian border guards who police parts of Armenia’s frontiers.

The outbreak of hostilities in the late 1980s prompted mass expulsions of hundreds of thousands of mostly Muslim Azeris from Armenia and Armenians, who are majority Christian, from Azerbaijan.

Peace talks began after Azerbaijan retook Karabakh by force in September 2023, prompting almost all of the territory’s 100,000 Armenians to flee to Armenia. Most now live in Armenia as refugees.

Both sides had said they wanted to sign a treaty to end the long-running conflict, but progress has been slow and relations tense.

Their 621-mile shared border is closed and heavily militarized.

In January, Azerbaijani President Ilham Aliyev accused Armenia of posing a “fascist” threat that needed to be destroyed, in comments that Armenia’s leader called a possible attempt to justify fresh conflict.