By Tom Miles and Mohammed Ghobari
GENEVA/SANAA, June 19 (Reuters) - U.N.-sponsored talks in Geneva on a ceasefire between Yemen's civil war parties ended on Friday without a deal as Saudi-led warplanes staged further strikes on the dominant Houthi armed faction and allies including elite Republican Guards.
More than 2,800 people have been killed since an Arab alliance launched air raids on March 26 to try to roll back the Iranian-backed Houthis' advances across much of Yemen and reinstate exiled President Abd-Rabbu Mansour Hadi.
U.N. special envoy Ismail Ould Cheikh Ahmed said that in five days of "proximity talks" -- in which he shuttled between factions who refused to sit at the same table -- the two sides agreed in principle on the need for a ceasefire and withdrawal of forces in keeping with U.N. Security Council Resolution 2216.
"There is a certain willingness from all the parties to discuss issues around a ceasefire accompanied by withdrawal ... I personally come out from these few days with a certain degree of optimism that we can achieve this (in further consultations) in the coming days," he told a news conference in Geneva.
"The (opposing) positions that as you know have been so strong in view of so many lives having been lost and where a government that is internationally recognized is outside the country. A lot of things didn't make it easy for us."
He said he would fly to New York on Sunday to brief the U.N. Security Council, where major powers also needed to sign off on his proposal for a force of civilian observers to monitor any truce and withdrawals on the ground.
Hadi's foreign minister, Reyad Yassin Abdullah, said the talks made no significant headway but there was room for more discussions, although no date had been scheduled for any.
"Unfortunately the Houthi delegation did not allow us to really reach all progress as we expected ... But it doesn't mean that we have failed," he told reporters.
Hadi's government has demanded that the Houthis, who are allied with Yemeni military units loyal to former president Ali Abdullah Saleh, pull out of cities captured since last September as a precondition for a ceasefire.
Yahya Duwaid of Saleh's General People's Congress said: "We had reason to be hopeful and optimistic...and we listened to the U.N. proposals today, but unfortunately, what they were proposing was not of the standard that we were looking for."
In Washington, U.S. State Department spokesman John Kirby told reporters that the Geneva talks were a useful start. "We have to expect that it could be a lengthy process," he said.
Yemen has been in upheaval since the Houthis surged out of their stronghold Saada province in the north and seized the capital Sanaa in September, a move they said was aimed at forcing Hadi to bring them into the government.
But the heavily armed Houthis, from the Zaydi branch of Shi'ite Islam, swiftly pushed into central and south Yemen including the major port of Aden, driving Hadi's government into exile in Saudi Arabia.
The group denies drawing military support from Shi'ite Iran and says it is fighting against corruption and Sunni Muslim al Qaeda militants who grabbed power in parts of the south during a 2011 uprising that spread anarchy and ousted Saleh.
Saudi Arabia, the world's top oil exporter, intervened militarily out of concern for what it sees as a growing Iranian sway in the Arabian Peninsula, but air strikes have yet to significantly reverse Houthi territorial gains.
In Sanaa, residents reported three air strikes early on Friday on the al-Sawad camp in a southern suburb where the command of the Republican Guards allied with Saleh and the Houthis is based.
Another three air strikes were reported in the Khawlan region, southeast of Sanaa, six on a camp housing the Houthi-allied 115th Infantry Brigade in the al-Hazm district of al-Jouf province, and three on Houthi positions on Aden's outskirts.
Residents gave no details on casualties, but the Houthis reported that nine civilians were killed in air strikes on the Razeh district of Saada province that is the Houthi heartland, bordering on Saudi Arabia.
The United Nations on Friday also launched a revised humanitarian appeal of $1.6 billion for the large number of Yemenis trapped or displaced by the conflict.
"Over 21 million people or 80 percent of the population are now estimated to be in need of some form of humanitarian aid and or protection," U.N. spokesman Jens Laerke told a news briefing. (Additional reporting Stephanie Nebehay and Lara Sukhtian in Geneva, Arshad Mohammed in Washington; Editing by Mark Heinrich)