0820 GMT March 23, 2019
The five states bordering the resource-rich Caspian Sea signed a deal on its legal status on Sunday in the Kazakh city of Aktau, easing regional tensions and potentially facilitating lucrative oil and gas projects.
The presidents of Iran, Azerbaijan, Kazakhstan, Russia and Turkmenistan signed the agreement on the world's largest inland body of water, which has been disputed since the 1991 collapse of the Soviet Union rendered obsolete agreements between Tehran and Moscow.
The host, Kazakh President Nursultan Nazarbayev said before the signing that the leaders were "participants in a historic event."
"We can admit that consensus on the status of the sea was hard to reach and not immediate, the talks lasted more than 20 years and called for a lot of joint efforts from the parties," Nazarbayev said.
Russian President Vladimir Putin, whose country was seen as driving the deal, said the convention had "epoch-making significance" and called for more military cooperation between the countries on the Caspian.
Sunday's summit was the fifth of its kind since 2002 but there have been more than 50 lower-level meetings since the Soviet breakup spawned four new countries on the shores of the Caspian.
The deal goes some way to settling a long-lasting dispute on whether the Caspian is a sea or a lake – which means it falls under different international laws.
While the convention refers to the Caspian as a sea, provisions in the agreement give it "a special legal status", Russian Deputy Foreign Minister Grigory Karasin told Kommersant daily earlier this week.
The Kremlin has said the convention keeps most of the sea in shared use but divides up the seabed and underground resources.
The agreement establishes rules for declaring each country's territorial waters and fishing zones. However, the delimitation of the seabed – which has caused most disputes – will require additional agreements between littoral nations, Iranian President Hassan Rouhani said Sunday.
Rouhani called the convention "a major document" but noted that it does not bring an end to all disagreements over the sea.
"Today we have a framework for actions in the Caspian Sea which was not the case before," Rouhani said in comments translated into English.
"But there are other issues to deal with in other meetings."
Nevertheless, Rouhani hailed a stipulation in the convention that prevents non-Caspian countries from deploying military forces on the sea.
"The Caspian Sea only belongs to the Caspian states," he said.
The Caspian is the largest inland body of water in the world. With around 70,000 cubic kilometers (16,800 cubic miles) of water, it is bigger by volume than both the North and Baltic seas.
One of the reasons the Caspian has been contested so fiercely by the five littoral states is its abundance of hydrocarbon reserves.
According to the US Energy Information Administration's latest estimate in 2012 there were 48 billion barrels of oil and 292 trillion cubic feet (8.3 trillion cubic metres) of natural gas in proved and probable reserves in the Caspian Sea's basins and its surrounding area.
The Caspian's largest oil field, Kazakhstan's Kashagan, has 13 billion barrels of recoverable reserves and is the biggest offshore oil and gas find since it was discovered in 2000.
Extracting the hydrocarbons from the water and on to the market has not always been easy though, and delays to the Kashagan project have led to it being nicknamed "cash all gone".
As home to the world's largest stocks of the giant Beluga sturgeon fish, the Caspian is also an important source of one of the world's priciest delicacies – caviar.
According to Guinness World Records, a kilogram of the costliest caviar from albino sturgeon off the coast of Iran regularly fetches over $25,000.
AFP, AP and Reuters contributed to this story.