* Rebels seize two northern towns
* Say president's concessions "too little, too late"
* Fear returns to the capital (Adds colour, quotes from Bangui in paragraphs 10-13)
By Paul Marin Ngoupana
BANGUI, March 21 (Reuters) - Rebels in the Central African Republic clashed with government forces on Thursday and threatened a renewed assault on the capital, after rejecting a last-ditch peace offer.
The SELEKA rebel group seized two towns, hundreds of kilometres north of the capital Bangui, dismissing an offer from President Francois Bozize to agree to some of their demands as "too little, too late".
The resumption of hostilities after more than two months of a ceasefire is a concern for former colonial power France.
It increased the number of its troops in the country to 600 last year to protect French citizens working there, many of them in the mining industry.
CAR has rich deposits of gold, diamonds and uranium but it remains among the least developed countries in the world.
SELEKA says Bozize has not honoured a January peace deal and had given him until Wednesday to do so.
State TV announced two presidential decrees on Wednesday that failed to assuage the rebels' grievances. The decrees freed political prisoners, lifted curfews and banned roadblocks manned by pro-Bozize militia.
"We no longer trust Bozize's promises. We are calling for him to resign, otherwise we will force him out," SELEKA spokesman Eric Massi said by telephone from France.
A government official confirmed that rebel forces had first taken Bouca, a town with an airport some 300 km (240 miles) north of Bangui, before seizing Batangafo, 100 km further to the north, later in the morning.
Massi said SELEKA's forces had encountered light resistance from government troops in Bouca. The rebels captured three soldiers and two government army vehicles there, he said, and were unopposed in Batangafo.
'TIME FOR PRAYER'
There were no immediate reports of other clashes but tensions ran high in Bangui over a possible return to fighting after rebels marched to the gates of the crumbling riverside capital in December.
Some residents stockpiled water and food, fearing clashes between the rebels and government troops could force them indoors or cut off supplies for days.
"This is the time for prayer and fasting so God can guide our brothers from anger to wisdom, to avoid the bloody path of weapons," said Celine Mayama, a women's group leader in Bangui, speaking on local radio.
Army trucks carrying troops trundled along the dusty streets of the city past normally buzzing cafes and boutiques, many of which were empty or shut by early afternoon.
CAR's current uprising is the latest in a long line of rebellions that have crippled the country. Conflicts in neighbouring Chad, Sudan and Democratic Republic of Congo have also undermined peace efforts.
The U.N. Security Council voiced strong concern about the situation on Wednesday and has called on all sides to abide by the January peace deal.
Chad's Foreign Minister Moussa Faki Mahamat said President Idriss Deby, in conjunction with his Congolese counterpart Denis Sassou-Nguesso, was seeking to organise a summit in Bangui to press for the peace accord to be implemented.
As well as freeing political prisoners and removing checkpoints, the insurgents have refused to disarm before the departure of foreign peacekeeping troops, including some 400 South Africans.
Clayson Monyela, a spokesman for South Africa's foreign ministry, said: "The troops are staying. They aren't going anywhere."
The insurgents also want senior army posts for their military leaders as agreed under January's power-sharing deal, which has already allowed rebels to enter the cabinet.
French nuclear energy group Areva has rights to the country's Bakouma uranium deposit. (Additional reporting by Richard Valdmanis in Dakar, John Irish in Paris and David Dolan in Johannesburg; Writing by David Lewis; Editing by Erica Billingham)