International watchdog group Human Rights Watch (HRW) has accused Azerbaijan of "repeatedly" using widely banned cluster munitions in residential areas in Nagorno-Karabakh since its war with Armenian forces over the enclave dramatically escalated last month.

HRW on October 23 cited four such incidents based on "an on-site investigation" in the breakaway Azerbaijani territory, which is mostly populated by ethnic Armenians and has been at the heart of post-Soviet Europe's longest running "frozen conflict."

The latest accusation of Baku's use of the banned weapons came just hours ahead of separate meetings in Washington between U.S. Secretary of State Mike Pompeo and the Azerbaijani and Armenian foreign ministers.

Hundreds of people have been killed since fighting flared on September 27, raising fears of a wider conflict in the South Caucasus drawing in Turkey, which backs Azerbaijan, and Russia, a key player in the region that has a military treaty with Armenia.

Optimism that the talks in Washington might quell the fighting appeared to dim on October 22 as the two sides engaged in new battles while leaders in Baku and Yerevan hardened their positions despite two Russian-brokered cease-fires between the Caucasus archfoes that both collapsed quickly after being agreed upon.

Cluster munitions are banned due to their unpredictable effects across a widespread area, as well as long-term dangers from unexploded submunitions. They can be dropped from the air or fired from the ground.

Both the Azerbaijani and Armenian sides have denied using cluster bombs while accusing the other of firing them.

HRW said it had not verified Azerbaijan's claims that the Armenian side had used cluster munitions, .

HRW said its observers personally inspected "remnants of the rockets, impacts, and remnants of submunitions that exploded, as well as dud submunitions" fired by Azerbaijani forces at several locations in Stepanakert.

Until recently, Stepanakert (Xankandi in Azeri) housed around 55,000 of the mountainous region's 150,000 or so people, but the city has been devastated by artillery and other fire in the current fighting.

HRW said it also saw photographs of similar evidence from at least one other town.

"The continued use of cluster munitions -- particularly in populated areas -- shows flagrant disregard for the safety of civilians," HRW quoted its arms-division director, Stephen Goose, who is also chairman of the Cluster Munition Coalition campaign to eradicate the weapons, as saying.

"Cluster munitions should never be used by anyone under any circumstances, much less in cities, due to the foreseeable and unacceptable harm to civilians," he added.

Both sides also have denied targeting civilians, but rights groups have warned against the use of banned munitions such as cluster bombs.

Amnesty International said on October 5 that its experts identified cluster munitions that appeared to have been fired by Azerbaijani forces into residential areas of Stepanakert in the early days of this flare-up.

"The use of cluster bombs in any circumstances is banned under international humanitarian law, so their use to attack civilian areas is particularly dangerous and will only lead to further deaths and injuries," London-based Amnesty International said in a statement.