New Delhi, Apr 13 (IANS): As the conflict intensified amid reports of Sudanese warplanes launching their first attack on a key South Sudanese town, India, which has around $2.5 billion oil investments across undivided Sudan, voiced concern and urged both sides to restart negotiations.

"The government of India is concerned with the recent developments in Sudan and South Sudan," external affairs ministry spokesperson Syed Akbaruddin said here.

"We again urge both sides to exercise restraint and recommence negotiations, which are being held under the African Union High Level Implementation Panel, to resolve all outstanding issues peacefully," said the spokesperson.

In the worst clashes since South Sudan gained independence last July, Sudanese aircraft are reported to have attacked the capital of the Unity state in the South.

On Wednesday, South Sudan is reported to have seized the town of Heglig, which has around half of Sudan's oil production. The conflict threatens to escalate as South Sudan's President Salva Kiir has said he will not withdraw his troops from the Heglig oilfield.

The escalation of the conflict takes place barely a fortnight after India sent special envoy Amarendra Khatua, an additional secretary in the external affairs ministry, to meet top officials in Khartoum and Juba March-end. In his meetings, Khatua impressed upon both sides to resolve all issues peacefully.

ONGC Videsh Limited, the overseas arm of India's state-owned oil major, has $2.5 billion investments in petroleum exploration and production in undivided Sudan as part of the Greater Nile Petroleum Operating Co., in which it owns a 25 percent stake.

The conflict between the two Sudans has affected OVL's commitment to supply 12,000 barrels of oil per day to Sudan, causing a loss of over $8 million over the last one month.

Both Sudan and South Sudan have, however, assured India that its oil investments spanning the borders are safe and will not be affected by the dispute over oil revenues between the two countries.