Armenia has warned that renewed hostilities over Azerbaijan's breakaway region of Nagorno-Karabakh could lead to "full-scale war" and the recognition of the territory as an independent state, while Baku has announced new fatalities among soldiers fighting separatist forces and threatened to expand its operations along the front line.
Azerbaijan's disclosure of three deaths brings to 15 the number of its soldiers reported killed amid renewed fighting in the South Caucasus mountain enclave, while 18 combat deaths have been reported by the Armenian-backed separatists.
The destruction of heavy military equipment has been claimed by both sides.
Armenian Defense Ministry said on April 4 that five people had been killed in an Azerbaijani drone attack on a bus carrying Armenian volunteers to Nagorno-Karabakh.
There was no immediate comment from the Azerbaijani side.
Azerbaijan and Armenia have been locked in a conflict over Nagorno-Karabakh for years. Nagorno-Karabakh, populated mainly by ethnic Armenians, declared independence from Azerbaijan amid a 1988-94 war that claimed an estimated 30,000 lives and displaced hundreds of thousands of people.
Diplomatic efforts to settle the conflict have brought little progress, and there have been sporadic flare-ups of fighting.
A day after the worst outbreak of violence in nearly two decades broke out early on April 2, Azerbaijan announced a unilateral cease-fire.
Karabakh military officials said the territory was ready to discuss the terms of a truce, but only in the context of "restoring former positions." The West, Russia, and Iran have appealed to all sides to exhibit restraint.
Speaking at an April 4 meeting with the ambassadors of the Organization for Security and Cooperation in Europe (OSCE), Sarkisian said international appeals would fall on deaf ears unless world powers forced the Azerbaijani leadership to abide by the 1994 truce that stopped the war.
Sarkisian said the OSCE must also bolster the cease-fire by increasing the number of his field observers deployed in the conflict zone and "urgently" introducing a mechanism for investigations of cease-fire violations.
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He warned that a further escalation of the fighting would be fraught with "unpredictable and irreversible consequences, including a full-scale war."
"I must point out here that I have repeatedly stated that if hostilities continue and become large-scale, the Republic of Armenia will recognize Karabakh's independence," the Armenian president added.
Azerbaijani President Ilham Aliyev said in a televised meeting of top advisers on April 2 that troops had achieved a "great victory," while vowing to "observe the cease-fire and after that we will try to solve the conflict peacefully.”
Each side has accused the other of targeting civilians.
On April 4, Azerbaijan's Foreign Ministry warned that "Armenia will bear the blame for possible counterattacks and retaliatory measures by Azerbaijan's armed forces."
"In the event of continued Armenian provocations, we will launch a full-scale operation along the entire front line, using all kinds of weapons," Azerbaijani Defense Ministry spokesman Vagif Dargahly told journalists.
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Separatist forces said Azerbaijani troops had "intensified shelling" of their positions in the region's northeast and southeast sections of the "line of contact" -- which effectively serves as a front line separating the opposing sides -- using mortars, rocket-propelled artillery, and tanks.
Armenian Defense Ministry spokesman Artsrun Hovhannisyan said the separatists "seriously advanced at certain sectors of the front line and took up new positions."
Azerbaijan dismissed the report. Its Defense Ministry said it was in control of several strategic heights in Karabakh that were captured by Azeri troops backed by tanks and heavy artillery on April 2.
The ministry said Azerbaijani forces responded to the Armenian fire by using the Russian-made TOS-1 heavy flamethrower system.
It also released a video purportedly showing the destruction of an Armenian command post in which it said an Armenian general and other high-ranking officers were killed.
The military of the unrecognized Nagorno-Karabakh republic denied the allegations, but said it had destroyed an entire Azerbaijani Army unit.
A legacy of the Soviet breakup known as a "frozen conflict," the Nagorno-Karabakh dispute has bedeviled regional and international leaders for years, with the United States, Russia, and France taking the lead in trying to reach a permanent settlement, and tamp down tensions.
Diplomats from the three states, grouped together in what's called the Minsk Group, said they would convene a full-meeting on April 5 in Vienna to discuss the breakdown of the 21-year-old cease-fire.
Meanwhile, Russia's Foreign Ministry said Russian Foreign Minister Sergei Lavrov and U.S. Secretary of State John Kerry had a phone conversation on April 4 and expressed "serious concern" over an escalation in the standoff.
The ministry added in a statement that both sides had called for a swift cessation in fighting over Nagorno-Karabakh.
U.S. State Department spokesman Mark Toner later confirmed that Kerry and Lavrov discussed the conflict on April 4.
"They did speak today via phone...to discuss efforts to secure an immediate end to the violence that has erupted along the Nagorno-Karabakh line of conflict and encourage both Armenia and Azerbaijan to...resume settlement talks under the auspices of the OSCE," Toner told reporters.
U.S. Vice President Joe Biden said on Twitter he had urged the presidents of Armenia and Azerbaijan, Serzh Sarkisian and Ilham Aliyev, to settle the conflict.
Biden wrote that he told Sarkisian and Aliyev that "a comprehensive settlement in Nagorno-Karabakh is critical for their stability, security, prosperity."