Egypt's president Abdel Fattah al-Sisi warned on Saturday that advances by Turkey-backed forces in Libya could prompt an Egyptian military intervention in the neighbouring country.
Forces loyal to the Tripoli-based Government of National Accord (GNA) have been pushing to take the strategic city of Sirte, which is under the control of eastern-based commander Khalifa Haftar, who is backed by Cairo.
Ankara said on Saturday that Haftar's forces must withdraw from Sirte before a lasting ceasefire can be implemented.
However in a televised address, Sisi said Sirte is a "red line" for Egypt, citing the need to protect its porous border as grounds for "direct intervention" in Libya.
"Any direct intervention by Egypt has become internationally legitimate, whether under the UN charter on self defence or based on the sole legitimate authority elected by the Libyan people: the Libyan parliament," Sisi said.
Like Haftar, the head of the Libyan parliament is based in the east of the country, where they are both supported by Egypt.
"Some think they can trespass on the Sirte or Al-Jufra frontline. This for us is a red line," Sisi said.
"If the Libyan people asked us to intervene, it is a signal to the world that Egypt and Libya share ... common interests, security and stability," he added.
Libya has been torn by violence, drawing in tribal militias, jihadists and mercenaries since the 2011 toppling and killing of longtime dictator Moamer Kadhafi in a Western-backed uprising.
The oil-rich country is split between rival administrations in the east and west and has recently become an international battlefield attracting increasing foreign involvement.
Egypt, Russia and the United Arab Emirates back Haftar, who since last year has sought to regain control over the west in an abortive attempt to seize Tripoli.
The GNA has recently made major military gains against Haftar's forces thanks to ramped up support from its backer Turkey.
The Egyptian president blamed "external forces" supporting "extremist militias and mercenaries" in Libya for blocking a peace initiative proposed by Cairo earlier this month.
Cairo's proposal called for a ceasefire in Libya, the withdrawal of foreign mercenaries, and disbanding of Libyan militias.
Earlier on Saturday, Sisi urged Egyptian troops to "be prepared to carry out any mission, inside our borders, or if necessary, outside our borders."
On Friday, the Arab League announced plans to hold an urgent virtual foreign ministers meeting on Libya this week following Egypt's request.
Libya's GNA said it would boycott the meeting as it "merely deepen the rift" between Arab governments on the conflict.