Military and maritime agreements between Turkey and Libya's government in Tripoli represent a "clear escalation" of the conflict in the country, the UN's Libya envoy said.
Ghassan Salame's remarks came as Egypt called for an urgent meeting on Tuesday of the Arab League to discuss developments in the neighbouring country.
Libya has been racked by conflict since a Nato-backed uprising toppled and killed dictator Muammar Qaddafi in 2011, with rival administrations in the east and the west vying for power.
In April, Field Marshal Khalifa Haftar, who is based in the east, ordered his Libyan National Army to seize the capital Tripoli from the UN-recognised Government of National Accord and allied militias.
Last week, Turkey's President Recep Tayyip Erdogan announced plans to send troops to support the GNA against the attack.
Ankara signed a security and military co-operation deal with the GNA in November, as well as an agreement on maritime jurisdiction in the Mediterranean.
The agreements signed between Ankara and the GNA represent "a clear escalation of the conflict" in Libya as more interests from outside the country become involved, Mr Salame told the French daily Le Monde.
"The past few weeks have been marked by a worsening of this outside interference," Mr Salame said.
"You had the two agreements, military and maritime, signed on November 27 by the GNA and Turkey, to which are added the declarations of the Turkish authorities [on sending troops].
"And you have, on the other side, several countries that support Haftar's war effort at the entrance to Tripoli."
The UN envoy, who took up the post in 2017, also deplored what he called the "internationalisation of the conflict".
Mr Salame noted the presence of "Arabic-speaking groups, probably from Syria, who are fighting alongside the GNA and the arrival of planes from Syria to the airport in Benghazi", in eastern Libya.
"We are facing an extremely dangerous situation where the credibility of the UN is at stake," he said.
Mr Salame said he was disappointed at the failure of the UN Security Council to pass a resolution calling for a ceasefire in Tripoli.
"All of this causes outside interference to multiply and get worse," he said.