Saad Hariri sent shock waves through Lebanon when he unexpectedly quit as prime minister a week ago, though on Sunday he rejected rumours he was under de facto house arrest in Riyadh, insisting he was "free" and would return home soon.
His resignation came as tensions rise between Riyadh and Tehran, which back opposing sides in power struggles from Lebanon and Syria to Yemen.
The EU's diplomatic chief Federica Mogherini said that a meeting of the bloc's foreign ministers in Brussels on Monday had voiced unanimous support for Lebanon's "unity and stability".
"We expect no external interference in this national agenda and we believe it is essential to avoid importing into Lebanon regional conflicts, regional dynamics, regional tensions that have to stay out of the country," Mogherini said, adding that she would meet Lebanese Foreign Minister Gebran Bassil in Brussels on Tuesday.
She said outside interference in Lebanon -- buffeted for decades by conflicts between bigger players in the region such as Iran and Syria -- was a "current and always existing threat".
French Foreign Minister Jean-Yves Le Drian said France was "worried by the situation in Lebanon" and wanted to see the government there "stabilise as quickly as possible".
"For there to be a political solution in Lebanon, it is necessary that all of the political leaders have total freedom of movement and that non-interference is a fundamental principle," Le Drian said as he arrived for the EU foreign ministers' gathering in Brussels.
German Foreign Minister Sigmar Gabriel said there was a danger of Lebanon falling back into "political and sometimes military confrontations".
"In order to prevent this we need especially the return of the current prime minister, reconciliation in the country and the prevention of influence from outside," he said.
"Lebanon has earned the right to decide on its fate by itself and not become a pinball of Syria or Saudi Arabia or other national interests," Gabriel said in Brussels.
Other Western countries have moved to express their support for Hariri, with US Secretary of State Rex Tillerson calling him a "strong partner".
Tillerson warned against "any party, within or outside Lebanon, using Lebanon as a venue for proxy conflicts or in any manner contributing to instability in that country".
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