By Moumine Ngarmbassa

N'DJAMENA, March 4 (Reuters) - Europe's military mission to Chad has suffered its first casualty and another soldier went missing after a patrol strayed into Sudan and clashed with Sudanese troops, European officials said on Tuesday.

The incident on Monday in the tri-border region of Chad, Sudan and Central African Republic is the most serious so far to befall the European Union force (EUFOR) that is still being deployed in eastern Chad.

The EUFOR mission of up to 3,700-strong, which has a United Nations mandate to protect refugees displaced by violence in Sudan's Darfur region, had already seen its deployment delayed last month by a Chadian rebel attack on N'Djamena.

Some analysts have questioned whether the force has the capacity to carry out its mission without getting sucked into Chad and Sudan's interlocking conflicts.

French Defence Minister Herve Morin said a French army sergeant and a soldier serving in the EU force in Chad strayed into Sudan on Monday while on a reconnaissance patrol and were fired upon by Sudanese soldiers.

One of the soldiers was wounded but managed to rejoin other EU forces who came to investigate. The other was missing.

The European Union and France had apologised to Sudan about what they called the unintentional and inadvertent crossing of the Sudanese border by a EUFOR vehicle.

Morin and EUFOR's operational commander, Lieutenant General Patrick Nash of Ireland, asked the Sudanese authorities to help find the missing soldier.

"EUFOR carries out its mandate in full respect of the sovereignty of the Sudanese border," Nash said in a statement released by EUFOR. The force's deployment remained unaffected.

Sudan welcomed the EU and French apologies.

Sudanese foreign ministry spokesman Ali al-Sadig said Khartoum has ordered army units in the border area to "intensify their efforts to locate the missing French soldier."

DEPLOYMENT RISKS

Bjoern Seibert, an analyst at Boston's Fletcher School of Law and Diplomacy, said the incident was not surprising given the lack of a clear demarcation line in the rugged border area.

It also showed Sudan may be concerned about possible French or European attempts to gather intelligence on its border.

"The incident highlights the potential for further border incidents, which could draw EUFOR deeper into the regional conflict," Seibert, the author of a study on the EU military deployment in Chad, said in an emailed comment.

The incident is embarrassing for EUFOR, as critics of the mission have already questioned its neutrality, because more than half of its troops are being provided by France, which already has soldiers and planes based in Chad.

French military intelligence and logistics support last month helped Chadian President Idriss Deby fight off the rebel attack on the capital N'Djamena. Chad accuses Sudan of backing the rebels, a charge denied by Khartoum.

But the critics say this context puts the EUFOR force at risk of being perceived as supporting Deby against Sudan.

Morin said the French soldiers "crossed the frontier without realising it". He said they were attacked immediately despite identifying themselves to the "apparently Sudanese" forces they encountered. "One of the two men collapsed, the other was able to get out of the jeep and take cover," Morin added.

Sudan's Sadig said earlier on Tuesday a group of "white soldiers" in a military jeep entered Sudan from Chad on Monday and fired at a Sudanese army checkpoint. He added that after the initial clash a larger force supported by a helicopter entered Sudan from Chad and clashed with the same checkpoint. "In this clash, a Sudanese civilian was killed." (For full Reuters Africa coverage and to have your say on the top issues, visit: http://africa.reuters.com/) (Reporting by Alaa Shahine in Khartoum, James Mackenzie in Paris and Pascal Fletcher in Dakar; Writing by Pascal Fletcher)

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