* U.N.'s Ban slams attack on "clearly marked helicopter"
* South Sudan army says regrets incident
JUBA, Dec 21 (Reuters) - South Sudan's army shot down a United Nations peacekeeping helicopter in the restive Jonglei state on Friday, killing the four Russian crew members onboard, U.N. and military officials said.
A U.N. source said the helicopter was on a reconnaissance mission in an area where the SPLA, South Sudan's army, has been fighting rebels led by David Yau Yau.
U.N. Secretary-General Ban Ki-moon strongly condemned the attack on the "clearly marked" helicopter and in a statement called "on the government of South Sudan to immediately carry out an investigation and bring to account those responsible for this act."
He demanded measures be put in place to prevent any further incidents in South Sudan, where the U.N. mission, known as UNMISS, was created after it seceded from Sudan in July 2011.
The U.N. Security Council echoed Ban's remarks, saying in a statement that it "strongly deplored" the incident, which it said was a "grave violation of the Status of Forces Agreement" between South Sudan and UNMISS and jeopardized the operations of the mission.
South Sudan's army first denied it had shot down the Russian helicopter but later said it had mistaken it for a Sudanese plane supplying Yau Yau rebels in Jonglei.
"We regret the incident," army spokesman Philip Aguer said, adding an artillery unit had spotted a plane landing in an area where Yau Yau forces were operating.
"We saw a white plane landing and asked UNMISS whether they had any flight in the area but they denied it. The army opened fire because it thought it was an enemy plane supplying Yau Yau with weapons." he said. "We later heard UNMISS had a flight there. They should have informed us."
U.N. peacekeeping spokesman Kieran Dwyer said in New York that since Sept. 27, 2011, there have been six other incidents involving detention, searching, and shooting at UNMISS aircraft and threatening of passengers and crew.
South Sudan often accuses Sudan of airdropping weapons to rebels in Jonglei.
Russia's Itar-Tass news agency quoted a source at the Russian embassy in South Sudan as saying the Mi-8 helicopter owned by Nizhnevartovskavia was working under a U.N. contract when it was downed.
Earlier this year, Russia said it would withdraw helicopters and personnel servicing the U.N. mission in South Sudan after voicing alarm at attacks on U.N. helicopters there.
In September, South Sudanese soldiers killed at least 10 troops when they shot and sank one of their own military riverboats in a remote region after mistaking it for an enemy craft, the army said.
South Sudan has been struggling since independence to build up state institutions in a country awash with weapons after decades of civil war with Khartoum ended with a 2005 peace agreement.
Human rights groups often accuse the SPLA, a loose group of former guerillas, of human rights violations and abuses. The army denies this.
Yau Yau, a former theology student, heads one of several militias fighting the government, which accuses Sudan of supporting them. Sudan denies this.
A shortwave radio station with links to the Yau Yau rebellion earlier this year said the group was fighting the government in reaction to abuses committed during the disarmament programme.
In a separate incident in the western Darfur region of neighboring Sudan that highlighted the risks blue-helmeted troops face in the region, four peacekeepers were killed by a comrade who opened fire, the U.N.-African Union Mission in Darfur.