* Three-week conflict cleaves country along ethnic lines
* Both sides agree to ceasefire but fighting continues
* World's youngest state risks full-blown civil war
By Aaron Maasho and Carl Odera
ADDIS ABABA/JUBA, Jan 2 (Reuters) - South Sudan's army (SPLA) advanced towards the rebel-controlled town of Bor on Thursday, its spokesman said, as both sides gathered in Ethiopia for peace talks to end three weeks of violence that has pushed the country towards civil war.
Both sides have agreed in principle to a ceasefire but neither has indicated when the fighting, which has killed more than 1,000 people and displaced nearly 200,000, will stop.
South Sudanese President Salva Kiir declared a state of emergency late on Wednesday in Unity state and Jonglei, whose capital Bor fell on Tuesday into the hands of militia loyal to former vice president Riek Machar.
International pressure for a deal is mounting. Neighbouring countries mediating between the two warring sides have warned that continued fighting could scupper talks.
"The rebels are moving south from Bor and the SPLA is moving towards Bor," SPLA spokesman Philip Aguer told Reuters.
Bor lies 190 kilometres (118 miles) to the north of the capital Juba by road. Analysts say control of Bor hands the rebels a territorial base relatively close to Juba, strengthening its negotiating hand.
Aguer declined to say how many government troops were moving towards Bor, where Nuer militias massacred Dinkas during an outburst of ethnic fighting in 1991. He estimated the number of rebels in Bor at between 4,000 and 7,000.
SPLA troops are also reorganising to win back Bentiu, the capital of Unity state in the country's north, Aguer said.
The peace talks in Ethiopia's capital Addis Ababa are meant to focus on finding ways to roll out and monitor the ceasefire, mediators said.
"Only half of the government's delegation are here. The rest are set to arrive this evening," said an Ethiopian official.
Each side would meet with Seyoum Mesfin, one of two mediators appointed by the east African bloc IGAD. "Then (they) hopefully proceed to face-to-face talks," the official added.
The clashes in South Sudan erupted on Dec. 15 and quickly spread to half of the country's 10 states, unsettling oil markets and raising fears of the conflict spilling over in an already fragile region.
Kiir has accused his long-term political rival Machar, whom he sacked in July, of starting the fighting in a bid to seize power. The conflict has split the country along ethnic lines, between Kiir's Dinkas and Machar's Nuer group.
Machar has denied Kiir's charge but he has taken to the bush and has acknowledged leading soldiers battling the government.
"We don't want to expose the people of South Sudan to a senseless war," South Sudan's Foreign Minister Barnaba Marial Benjamin said on Wednesday.
The White House raised the pressure for talks this week, saying there would be accountability for any atrocities and war crimes.
The United Nations Mission in South Sudan said ethnic-based atrocities, often carried out against civilians by uniformed men, had taken place across the country. (Additional reporting and writing by Richard Lough in Nairobi; Editing by Gareth Jones)