A sharp military escalation in Libya, where fighters led by eastern commander Khalifa Haftar have been battling the forces of the internationally recognized government, could risk igniting a direct conflict among the foreign powers that have poured in weapons and fighters in violation of an arms embargo.
Sisi warned last week that Egypt would not stand idle if there was a threat to national security in Egypt and its western neighbor, Libya.
Egypt, alongside the United Arab Emirates and Russia, backs Haftar, who abandoned an offensive on the capital Tripoli last month against forces backed by Turkey.
Libyan lawmakers allied to Haftar asked Cairo this month to intervene militarily to counter Turkey.
The Egyptian parliament said in a statement on Monday's vote that troops would be defending national security on a "strategic" western front, without giving details, any time frame or naming Libya directly. It also did not mention Turkey.
Egyptian state TV later ran banners on the screen saying: "Egypt and Libya, one people, one fate."
SISI SPEAKS TO TRUMP
Shortly before the vote, Sisi and US President Donald Trump spoke by phone.
"The two leaders affirmed the need for immediate de-escalation in Libya, including through a ceasefire and progress on economic and political negotiations," the White House said in a statement.
Trump also told French President Emmanuel Macron by phone that the conflict "has been exacerbated by the presence of foreign forces and arms," the White House said. UN spokesman Stephane Dujarric told reporters: "There is no military solution to the current crisis in Libya and there must be an immediate ceasefire."
Egypt is concerned about instability in Libya and Turkey's support for Haftar.
The Tripoli government's forces have moved closer to the central city of Sirte, which they hope to recapture from Haftar's Libyan National Army and is the gateway to oil exporting ports held by the LNA. Sisi has declared the Sirte front line a red line for Egypt.
Egypt has backed Haftar since 2014, when he first assembled former soldiers from late leader Muammar Gaddafi's army and tribesmen to fight Islamist militants in the east. Libya has been in chaos since 2011, when Gaddafi was ousted.
Egyptian officials often refer to armed groups in Tripoli and western Libya as "militias" and see Haftar as leading a "counter-terrorism force."
The head of the defense committee in the eastern-based Libyan parliament, Talal al-Mayhoub, said the Egyptian parliamentary move was the first step towards activating an Egyptian-Libyan defense pact.