Armenia said on Friday that Azerbaijan had failed to fulfil a promise in full to withdraw troops that had crossed the border in a disputed incident, and that it had sought Russia's military help.

Six months after the worst fighting in decades between Azeri and ethnic Armenian forces, the border incident this week has demonstrated the fragility of a Russian-brokered ceasefire that halted the conflict.

Armenia has accused Azerbaijan of sending troops across the frontier. Azerbaijan has denied this and said its forces only defended their side of the frontier.

"Yesterday an agreement was reached that today Azerbaijan's armed forces should leave Armenian territory," Interfax news agency quoted Armenia's caretaker prime minister, Nikol Pashinyan, as saying at a meeting about the border.

"This agreement has been partially fulfilled; there are territories that the Azerbaijanis did indeed leave. But, since the agreement was not fulfilled completely, today I have approached the president of Russia...for military assistance."

Azerbaijan's foreign ministry said the leadership of its border guards had met with the Armenian side on Friday to discuss tensions at the border. It gave few further details. While such a meeting would not be extraordinary - border guards from the two sides met as recently as Wednesday - it would be a sign that communications remain open.

Armenia's Pashinyan called Russian President Vladimir Putin late on Thursday. In a statement about the phone call, the Kremlin said Putin had told Pashinyan that the ceasefire agreement should be strictly upheld, and that Pashinyan had in turn backed "solving all problems that arise through peaceful, political and diplomatic means".

"Of course, the Armenian side expressed extreme concern over the situation at the border. This concern was shared by President Putin," Kremlin spokesman Dmitry Peskov said on Friday.

U.S. State Department spokeswoman Jalina Porter said Washington expected Azerbaijan to immediately pull back its forces and "cease further provocation."

"Military movements in disputed territories are irresponsible and they're also unnecessarily provocative," Porter said on a call with reporters, adding that border demarcation issues should be resolved through negotiation and discussion.

Last year's fighting saw Azeri troops drive ethnic Armenians out of swathes of territory they had controlled since the 1990s in and around Azerbaijan's Nagorno-Karabakh region. It was celebrated in Azerbaijan as a victory, while viewed in Armenia as a debacle. Pashinyan resigned after a stand-off with his military, remaining in office as a caretaker until elections next month.

Russia sent peacekeepers to the area last year to help enforce the ceasefire, and also has a military base in Armenia. It has strong ties and a mutual defence pact with Armenia, but is also on friendly terms with Azerbaijan.