China has warned India to halt joint energy projects with Vietnam in the South China Sea even as Beijing takes steps to reduce tensions with Hanoi and co-

operate more closely in oil and gas exploration in the disputed waters.

In a joint statement issued on Saturday at the conclusion of a high-profile visit to China by Nguyen Phu Trong, the head of Vietnam’s ruling Communist party, Beijing and Hanoi said they would “actively boost co-operation” in offshore oil and gas exploration and exploitation as well as speed up negotiations to find a peaceful settlement to their long-running dispute.

But Beijing also publicly castigated Hanoi and New Delhi for an agreement in which India’s state-owned Oil & Natural Gas Corp will work with its counterpart, Petrovietnam, to develop oil and gas assets in parts of the South China Sea that Vietnam claims.

A front-page editorial published in an important official Chinese newspaper warned that “India’s energy strategy is slipping into an extremely dangerous whirlpool”, and, Reuters reported, said that India should “turn around at the soonest opportunity and leave the South China Sea”.

The editorial in China Energy News, published by official Communist party mouthpiece, the People’s Daily, also said “challenging the core interests of a large, rising country for unknown oil at the bottom of the sea will not only lead to a crushing defeat for the Indian oil company, but will most likely seriously harm India’s whole energy security and interrupt its economic development”.

On Friday, Liu Weimin, of China’s foreign ministry, said China had “indisputable sovereignty” over the waters in question and warned off India, saying New Delhi and Hanoi should “instead take positive steps to ensure peace and stability in the South China Sea”.

The positive mood music from Mr Trong’s trip to China last week, where he met his counterpart, Hu Jintao, and other leaders, was interrupted by Vietnam’s decision to sign an exploration agreement with India on Wednesday, during a visit by Vietnam’s president Truong Tan Sang.

The communist neighbours appear keen to repair relations, which fell to the lowest point in recent years in May and June after Hanoi claimed Chinese ships had sabotaged Vietnamese-chartered oil exploration vessels that were operating within its exclusive economic zone.

China claims the South China Sea in its entirety, rejecting partial claims by Vietnam, Brunei, Malaysia, the Philippines and Taiwan over the area, which may contain large oil and gas reserves and is traversed by significant trade routes.

Vietnam is trying to find a balance between assuaging its neighbour China and encouraging powers such as India and the US to help keep a check on Beijing’s grand maritime claims and increasing naval might.

One senior western diplomat in Hanoi warned it would be difficult for China and Vietnam to live up to the rhetoric of their latest agreement. “It’s a short-term fix to reduce tensions before things heat up again,” he said.