Turkey, Iran and Russia condemned Friday the U.S.' move to recognize Israeli sovereignty over occupied Syrian Golan Heights in a joint statement after the 12th round of Astana meetings on Syria in Nur-Sultan, the recently-renamed Kazakh capital.

The three guarantor countries of the Astana process on Syrian peace reaffirmed their strong commitment "to the sovereignty, independence, unity and territorial integrity of the Syrian Arab Republic and to the purposes and principles of the U.N. Charter."

On March 25, U.S. President Donald Trump signed a presidential proclamation officially recognizing the highlands on the border with Syria as Israel's territory.

Israel seized the Golan Heights from Syria during the 1967 Six-Day War. It occupies roughly two-thirds of the wider Golan Heights as a de facto result of the conflict. It moved to formally annex the territory in 1981 -- an action unanimously rejected at the time by the U.N. Security Council.

The guarantor countries underscored that the U.N. principles should be universally respected and that no actions, no matter by whom they were undertaken, should undermine them.

"In this regard they strongly condemned the decision of the U.S. Administration to recognize Israel's sovereignty over the occupied Syrian Golan Heights, which constitutes a grave violation of international law, particularly the U.N. Security Council resolution 497, and threatens peace and security in the Middle East," read the statement.

The first meeting of the Astana process was held in Turkey in January 2017 to bring all warring parties in the Syrian conflict to the table to facilitate U.N.-sponsored peace talks in Geneva. The Astana talks support the establishment of the U.N.-backed constitutional committee in Syria as a way to find a political solution. The planned constitutional committee, including representatives from the opposition, regime and guarantor countries, will be tasked with writing and establishing Syria's post-war constitution, which is seen as a stepping-stone to elections in the war-torn country.

The 11th round of the talks in the Astana format concluded in December with a decision to step up joint efforts to prevent violations of the cease-fire in Idlib.

In the joint statement following the 11th round of talks, it was stressed that the parties "examined in detail the situation in the Idlib de-escalation area and reaffirmed their determination to fully implement the Memorandum on the Stabilization of the Situation in the Idlib de-escalation area from Sept. 17, 2018."

"In this regard, they expressed their concern with the ongoing violations of the cease-fire and declared that, as guarantors of the cease-fire, they would step up their efforts to ensure observations, including by enhancing work of the Joint Iranian-Russian-Turkish Coordination Center," the statement added.

Syria has only just begun to emerge from a devastating conflict that began in early 2011, when the Bashar Assad regime cracked down on demonstrators with unexpected ferocity.

According to U.N. figures, hundreds of thousands of civilians have been killed or displaced in the conflict, mainly by regime airstrikes in opposition-held areas.