* U.N. talks aimed at pressuring Syrian government, rebels
* EU aid chief says Iran and Russia helping aid effort
* Russia cites "major breakthrough" in Syria's attitude
GENEVA, Dec 19 (Reuters) - Iran and Russia have used their influence on Syrian President Bashar al-Assad's government to get visas for U.N. aid workers and improve access for convoys in Syria, but much remains to be done, senior European officials told Reuters on Thursday.
U.S. officials were more cautious, saying that "all voices" taking part in secretive U.N. talks aimed at bringing food and medical supplies to civilians in out-of-reach areas are critical. There was limited progress in recent weeks, they said.
U.N. humanitarian chief Valerie Amos chaired a meeting in Geneva attended by some 20 states on Thursday to pressure the government and opposition to allow in more food and medical assistance, especially to people in besieged areas.
Saudi Arabia, Qatar and other Gulf states with links to rebel forces were urged to smooth deliveries, officials said.
"The important thing is to put pressure on the parties to the conflict to actually open up," Claus Sorensen, director-general of the European Union's humanitarian aid department ECHO, told Reuters in an interview after the high-level meeting.
Tehran and Moscow sent delegations to the talks, the second in a month. Neither Syrian officials nor rebels were invited.
Sorensen, asked about the role of Syria's allies Iran and Russia in the effort, told Reuters: "They are actually addressing things, I am grateful for what they have already done. It's not just Russia and Iran, but also Qatar and Kuwait."
He declined to give details, citing confidentiality rules.
Another European official said of Iran and Russia: "They have sought to facilitate movement of aid convoys and help with issues around visas. They have done some specific things and we would like them to do more because people are dying."
The U.N. Security Council issued a presidential statement on Oct. 2 calling for protection of civilians, demilitarisation of schools and hospitals, and improved access for aid workers.
Nancy Lindborg, USAID assistant administrator, welcomed a "slight shift" by Damascus in granting visas and a green light for cross-border U.N. assistance from Jordan, Lebanon, and Iraq.
"The Syrian regime, however, has the power to open besieged areas for humanitarian access tomorrow simply by taking the actions on the ground as outlined in the presidential statement," she told a news briefing.
An estimated 250,000 Syrians are living under siege, most of them encircled by government forces, but also including 45,000 in two northern towns besieged by rebels.
Some 9.3 million people inside Syria need humanitarian aid, Amos said on Monday, launching a record $6.5 billion appeal for Syria and its neighbours hosting refugees.
RUSSIA CITES BREAKTHROUGH
Russia's delegation to Thursday's meeting spoke of a "major breakthrough" with the Syrian government paying more attention to humanitarian needs and facilitating delivery, diplomats said.
Bureaucratic procedures had been simplified for registering cargo, a rule requiring two ministers to approve aid shipments had been lifted and some 50 visas were granted to aid workers in the last few weeks, a Russian official was quoted as saying.
The Syrian government was taking "a lot of positive steps and deserves to be encouraged", he said.
Agencies had delivered aid to hard-to-reach areas including those controlled by rebel groups in Deraa, the Old City of Homs and Hama province, he said. But militants had mined some hospitals and denied roads being used for aid deliveries.
U.S. officials cited some progress in granting of visas for aid workers but noted they must be accompanied by U.N. security.
"The U.N. security service has not gotten their visas, still," Anne C. Richard, Assistant U.S. Secretary of State, told the news briefing. (Reporting by Stephanie Nebehay; Editing by Mike Collett-White)