Itar-Tass news agency said Mr Ter Petrosian invoked a collective security pact signed in May by six members of the Commonwealth of Independent States. It obliges each to help the rest if attacked.
Mr Ter Petrosian, in a telegram to leaders of the other five states, said: 'Aggression has been committed against a state which is a member of the CIS and the system of collective security.'
He called on the heads of state to 'fulfil their obligations before the Republic of Armenia by using political, military or other methods to prevent a war'.
The appeal, the first time a member state has invoked the security treaty, was prompted by major military gains by Azerbaijan in the four-year conflict between the two former Soviet republics. More than 2,000 people have died in the fighting.
Reports from both sides said Azeri forces had captured Artsvashen, a pocket of Armenian territory within western Azerbaijan. Azeri reports said the armed forces had 'liberated' the town, destroying enemy tanks and weaponry and killing 300 Armenian brigands. Armenian reports mentioned no dead but said 29 people were missing without trace.
Mr Ter Petrosian's telegram described the attack on Artsvashen and the alleged Azeri bombing of Armenian territory as 'the beginning of undeclared war'.
The conflict between Christian Armenia and Muslim Azerbaijan is the deadliest of the many ethnic disputes left behind by the collapse of the Soviet Union. Frequent peace missions have ended in deadlock, and members of the Commonwealth of Independent States are unlikely to greet the Armenian request with much enthusiasm.
Six CIS members, Russia, Armenia and four central Asian republics, created the defence union last May at a summit in the Uzbek capital of Tashkent. Azerbaijan refused to join.
Armenia and Russia have discussed deploying CIS or international peace-keeping troops to halt the fighting, which has spread from the small disputed enclave of Nagorny Karabakh to engulf border regions as well. Azerbaijan has resisted the peace-keeping proposal as threatened interference.