Warlord Khalifa Haftar's visit comes as world powers step up efforts for a lasting ceasefire, nine months after Haftar's militia launched an assault on the Libyan capital Tripoli.
Greece encouraged Libyan warlord Khalifa Haftar to be constructive at an upcoming conference in Berlin aimed at ending the war over Tripoli, its foreign minister said on Friday after meeting the commander in Athens.
"We encouraged the commander to take part with a constructive spirit in Berlin's procedure and try ... to achieve a ceasefire and the restoration of safety in Libya," Greek Foreign Minister Nikos Dendias told reporters after the meeting.
He said Athens should have been a participant in the Berlin conference on Libya on Sunday.
Haftar was in Athens on Friday, just two days ahead of a peace conference in Berlin which he and the head of Tripoli's UN-recognised government Fayez al Sarraj are expected to attend.
Russian President Vladimir Putin will take part in Sunday's talks, Kremlin confirmed. The peace talks in Berlin will focus on stopping the fighting and launching a "broad political dialogue" under the auspices of the United Nations, the Kremlin added in a statement.
Turkish President Recep Tayyip Erdogan, who earlier with Putin announced a Libya ceasefire, will also be in attendance. Turkey and Libya's Government of National Accord (GNA) signed a maritime deal and military cooperation deal in November.
The interim ceasefire that came into force on Sunday has mostly held, with the rival Tripoli-based Government of National Accord (GNA) blaming Hafter for violations.
Libya has been in turmoil since former dictator Muammar Gaddafi was killed in a 2011 NATO-backed uprising.
Last April, an assault on Tripoli by Haftar's militia sparked fighting that has killed more than 280 civilians and 2,000 fighters, displacing thousands.
Haftar's Russia ties
In another statement, the Russian presidential administration said Haftar had thanked Putin for his efforts to end fighting in the war-ravaged country.
"Vladimir Putin, my dear friend!" Haftar said in a letter to Putin, quoted by the Kremlin.
"I express my personal gratitude and appreciation for the efforts of the Russian Federation to bring about peace and stability in Libya," Haftar was quoted as saying.
Earlier this week, Libya's warring sides –– Haftar and the head of Tripoli's UN-recognised GNA Fayez al Sarraj –– attended ceasefire talks in Moscow but Haftar left without signing a permanent ceasefire.
But German Foreign Minister Heiko Maas visited Haftar's eastern Libya stronghold of Benghazi on Thursday to persuade him to join the Berlin conference.
Haftar met Greek Foreign Minister Nikos Dendias after his arrival on Thursday. He is also set to meet Prime Minister Kyriakos Mitsotakis.
Greece is seeking to take part in the Berlin talks but has yet to be invited.
Athens is vehemently opposed to the Turkish deal with Libya, which claims much of the Mediterranean for energy exploration, conflicting with rival claims by Greece and Greek Cypriot Administration.
Haftar, Sarraj and Berlin talks
He "wants to contribute to the success of the Libyan conference in Berlin and is in principle ready to participate in it," Maas tweeted, calling it "the best chance in a long time" for peace.
But Sarraj, whose GNA did sign up to a permanent ceasefire deal in Moscow, cast doubt over Haftar's intentions after he refused to also sign.
Haftar "has chosen not to sign the agreement and asked for a delay", Sarraj said, calling that "an attempt to undermine the Berlin conference before it starts".
US Secretary of State Mike Pompeo will also take part in the truce efforts, the State Department said on Thursday.
The conference will aim to agree six points including a permanent ceasefire, implementation of a much-violated UN arms embargo and a return to political efforts for peace, Guterres said.
Turkish troops have been deployed to support the GNA, while Russia, despite its denials, is suspected of supporting Haftar with weapons, money and mercenaries.
Some 11 countries and several international organisations are set to attend along with the Libyan parties.
The fighting has spurred a growing exodus of migrants, many embarking on rickety boats towards Italy.
Nearly 1,000 intercepted at sea have been forced to return to the war-ravaged country since January 1, mostly ending up in detention, the UN's International Organization for Migration said Tuesday.