The US has expressed concern over Georgian reports of a "military buildup" near the administrative boundary (ABL) line of the Russia-backed region of South Ossetia.

Tbilisi said it had observed a "mobilisation of military equipment and personnel" in the separatist region after an ultimatum issued by the breakaway territory expired.

Georgia warned on Friday of the risk of a "serious confrontation" with South Ossetia after it demanded that Georgian authorities remove a checkpoint on its disputed boundary.

The US, Russia, and EU called for restraint after a demand from South Ossetia that Georgia dismantles a wooden observation post by August 30 went unmet.

"We call on the Russian Federation to utilize all available channels to prevent further escalation of the situation along the ABL," a US State Department spokesperson said.

"We express serious concern about the escalation of tension on the border of South Ossetia and Georgia," Russia's Ministry of Foreign Affairs said in a statement, urging "the parties to exercise restraint".

Russia cited the "backdrop of Tbilisi’s repeated refusal of Tskhinval’s proposal to begin negotiations on delimitation and demarcation of the border" as the reason the "South Ossetian side is forced to carry out all work on its engineering equipment unilaterally ... aimed at creating safe conditions for the life of the population, suppressing various incidents."

"Over the past few months, Tbilisi has deliberately escalated tensions on the border through provocative campaigns and political propaganda campaigns supported by Western countries," it added.

At a meeting of Incident Prevention and Response Mechanism (IPRM), mediated by the EU and the Organisation for Security and Co-operation in Europe (OSCE) on Thursday, South Ossetia demanded that Georgia dismantles the checkpoint before 5 am CEST on Friday, AFP cited one of the delegates at the meeting as saying.

"If this condition is not met, the government (of South Ossetia) will take all legal measures to ensure the security of the South Ossetian people and protect the state border," South Ossetian delegate to the IPRM Yegor Kochiev said in televised remarks.

Carl Hartzell, the EU's ambassador to Georgia, said early on Friday in a tweet that the "confrontational language" used at IPRM meeting was "unacceptable."

"We expect all sides to show maximum restraint and use their influence to ensure (the) situation doesn't escalate," he wrote.

Why are tensions building?

In the wake of spiralling tensions between Tbilisi and Moscow over Georgia's aspirations to join the EU and NATO, the disputed boundary line has become contentious.

Read more Georgia wants to be a part of Europe, president tells Euronews

Russia and Georgia fought a war in 2008, which claimed the lives of hundreds on both sides, over South Ossetia and Abkhazia, a further breakaway region.

Read more Europe's forgotten war: The Georgia-Russia conflict explained a decade on

After a French-mediated ceasefire put an end to the conflict, Russia recognised both South Ossetia and Abkhazia as independent states stationing permanent military bases there.

Georgia and its partners in the West labelled the move an "illegal occupation" in an area that represents around 20% of Georgia's sovereign territory.