* Mine planted by Syrian government forces, say opposition
* Violence continues despite calls for Eid al-Adha ceasefire
* ICRC asks "all authorities" for access to besieged town
* OPCW says conducted verification activities at 11 sites (Adds comment by U.N. chief Ban Ki-moon)
BEIRUT, Oct 16 (Reuters) - Twenty-one people, including four children and six women, were killed when a minibus hit a mine in the southern Syrian town of Noa on Wednesday, the Syrian Observatory for Human Rights monitoring group said.
Opposition activists told the Observatory the minibus drove over a mine planted by forces loyal to President Bashar al-Assad. There was no immediate comment from the government.
The explosion was reported in rebel-held territory in Deraa province but army troops are also stationed in the nearby base of Tel al-Jumaa, which is besieged.
The British-based Observatory, which is opposed to Assad, reported clashes in most provinces on Wednesday and said warplanes had been deployed to the eastern desert city of Deir al-Zor.
The group said at least 27 government soldiers had been killed during intense clashes in Deir al-Zor over the past two days although rebels gave a figure more than double that.
Assad's forces are battling an uprising that grew out of protests against his family's four-decade grip on the country.
Violence has continued in recent days despite pleas from Arab and Muslim organisations for a ceasefire to mark the Muslim festival of Eid al-Adha.
Pro- and anti-government militias have fragmented the country into fiefdoms, with hardline Islamist rebels fighting Kurds and other opposition groups.
The Observatory reported that 29 jihadist fighters, some from the al Qaeda-linked Islamic State of Iraq and the Levant, and 12 Kurdish militants were killed during clashes on Tuesday in the northeast province of Hasaka.
It is increasingly difficult for international aid workers to operate in the lawless rebel-held northern provinces.
Six Red Cross workers and a colleague from the Syrian Arab Red Crescent were abducted on Sunday after delivering medical supplies in the northern province of Idlib. Four were released the next day.
Aid workers say the Damascus government has also placed barriers on working in Syria such as rejecting visas or preventing convoys from entering certain areas.
Syrians complain aid is not getting through to many areas of the country, especially to residents living in rebel-held territory who say the government is restricting access.
ICRC spokesman Ewan Watson said his group was asking "all authorities for access" to Mouadamiya southwest of Damascus, where he estimated 10,000 civilians were trapped.
"We are concerned about the situation in the besieged town of Mouadamiya, under siege for 10 months. We have not been able to enter and suspect there are medical care needs and possibly of essential aid like food and water," he said in Geneva.
'DANGEROUS AND UNPREDICTABLE'
Earlier this month the United Nations Security Council urged the Syrian government to boost aid access by allowing cross-border deliveries and called on combatants in the civil war to agree humanitarian pauses in fighting.
"It is vital to turn those strong words into action," U.N. Secretary-General Ban Ki-moon told a news conference on Wednesday. "The humanitarian situation is worsening."
Assad has agreed to a U.N.-endorsed mission to eliminate his chemical weapons, a move widely seen as a concession to avoid U.S. military strikes after an August sarin gas attack in Damascus that killed hundreds in rebel-held areas.
Ban appointed Sigrid Kaag, of the Netherlands, on Wednesday as the special coordinator for the Organisation for the Prohibition of Chemical Weapons/United Nations joint mission.
Kaag, who speaks Arabic, is currently working for the United Nations Development Programme and has previously worked for the U.N. children's agency UNICEF, Ban said. She will be based in Cyprus report to Ban and OPCW Director-General Ahmet Üzümcü.
Experts from the OPCW have visited 11 of a total of around 20 declared sites where they are due to oversee the destruction of Syria's chemical arsenal and production facilities, the organisation said on Wednesday.
The OPCW - which last week was awarded the Nobel Peace Prize - said in a statement its activities so far included "critical equipment destruction at six sites" as well as some rockets designed for use with chemical weapons.
The most complex stage of their work, the destruction of chemical agents and precursors, has yet to start and the teams will likely have to visit at least one site - near the northern town of Safira - where fighting is continuing.
"We have no illusions over the challenges ahead. The situation in Syria remains dangerous and unpredictable. The cooperation of all parties in Syria is required," Ban said.
He said international Syria mediator Lakhdar Brahimi would travel to the region in coming days to meet "key parties" to push for a Syria peace conference, which the United Nations aims to hold in Geneva in mid-November. Brahimi's deputy Nasser al-Kidwa will also travel to Turkey shortly to meet with the Syrian opposition, Ban said. (Reporting by Oliver Holmes in Beirut, Stephanie Nebehay in Geneva, and Michelle Nichols at the United Nations; Editing by Angus MacSwan and David Brunnstrom)