At least 49 Armenian and 50 Azerbaijani soldiers were killed, each side has said, blaming the other for the renewed fighting.
It prompted an appeal for calm from Russian President Vladimir Putin.
Azerbaijan used artillery, mortar and small arms in its attack on Wednesday morning, the Armenian Defence Ministry said.
“The situation on the Armenian-Azerbaijani border remained tense,” it said, restating Armenia's position that Azerbaijan launched aggression on its sovereign territory.
On Tuesday, UN Secretary General Antonio Guterres called on the countries to reduce tensions.
“The secretary general is deeply concerned about reports of renewed fighting along the Armenian-Azerbaijani border,” Mr Guterres's spokesman Stephane Dujarric said on Tuesday.
The UN chief urged both sides to take immediate steps to exercise maximum restraint and resolve any outstanding problems through dialogue, Mr Dujarric said.
The fighting erupted minutes after midnight, with Azerbaijani forces unleashing an artillery barrage and drone attacks in many sections of Armenian territory, according to Armenia’s Defence Ministry.
It said shelling grew less intense during the day, but Azerbaijani troops were trying to advance into Armenian territory.
Azerbaijan’s Foreign Ministry said it was responding to a “large-scale provocation” by Armenia late on Monday and early on Tuesday. It said Armenian troops planted mines and fired on Azerbaijani military positions.
The two countries have been locked in a decades-old conflict over Nagorno-Karabakh, which is part of Azerbaijan but has been under the control of ethnic Armenian forces backed by Armenia since a separatist war there ended in 1994.
Azerbaijan reclaimed large parts of Nagorno-Karabakh in a six-week war in 2020 that killed more than 6,600 people and ended with a Russia-brokered peace deal. Moscow deployed about 2,000 troops to the region to serve as peacekeepers under the deal.
The Russian Foreign Ministry on Tuesday urged both parties “to refrain from further escalation and show restraint”.
Moscow has engaged in a delicate balancing act in seeking to maintain friendly ties with both former Soviet nations. It has strong economic and security ties with Armenia, which hosts a Russian military base, and has also been developing close co-operation with oil-rich Azerbaijan.
The international community also called for calm. The UN Security Council scheduled closed consultations on Wednesday to discuss the renewed fighting.
Armenian Prime Minister Nikol Pashinyan called Russian President Vladimir Putin and later had calls with French President Emmanuel Macron, European Council President Charles Michel and Iranian President Ebrahim Raisi.
Russian Foreign Minister Sergey Lavrov spoke by phone to his Azerbaijani counterpart, Jeyhun Bayramov.
US Secretary of State Antony Blinken spoke with both Mr Pashinyan and Azerbaijan’s President Ilham Aliyev.
Washington has a special envoy in the region, Mr Blinken said.
“My hope is that we can move this from conflict back to the negotiating table and back to trying to build a peace,” he said.
Speaking in parliament early on Tuesday, Mr Pashinyan accused Azerbaijan of having had an uncompromising stance at recent EU-brokered talks in Brussels.
Armenia said the Azerbaijani shelling on Tuesday damaged civilian infrastructure and wounded an unspecified number of people.
On Facebook, Mr Aliyev expressed condolences “to the families and relatives of our servicemen who died on September 13 while preventing large-scale provocations committed by the Armenian armed forces in the direction of the Kalbajar, Lachin, Dashkasan and Zangilan regions of Azerbaijan”.
Turkey, an ally of Azerbaijan, also placed the blame for the violence on Armenia with President Recep Tayyip Erdogan expressing support for Mr Aliyev and saying his country and Azerbaijan were “brotherly … in all matters”.
The governor of Gegharkunik province, one of the regions that came under Azerbaijani shelling, said there was a 40-minute lull in the fighting, apparently reflecting Moscow’s attempt to negotiate a truce, before it later resumed.
The governor, Karen Sarkisyan, said four Armenian troops in his region were killed while another 43 were wounded by the shelling.
The Armenian government said it would officially ask Russia for assistance under a friendship treaty between the countries, and also appeal to the UN and the Collective Security Treaty Organisation, a Moscow-dominated security alliance of former Soviet nations.
Kremlin spokesman Dmitry Peskov refrained from commenting on Armenia’s request but said during a conference call with reporters that Mr Putin was “taking every effort to help de-escalate tensions”.