"Relevant cooperation is in strict accordance with the two countries' international responsibility and is for peaceful aim. We also receive the supervision of the International Atomic Energy Agency (IAEA)," Chinese Foreign Ministry spokesperson Hua Chunying said.
Both India and United States conveyed their objections to China's move, saying that it violated the strict norms set by the 46-member Nuclear Suppliers Group (NSG).
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The issue may also figure during Prime Minister Manmohan Singh's visit here next week.
China and Pakistan, in principle were engaged in civilian energy cooperation in recent years, Hua said.
Recent reports from Pakistan said that two big reactors would be built in Karachi by China, with construction beginning as early as next month.
There is no direct confirmation of the agreement to build the new plants. However, in March this year, China indirectly confirmed reports of signing an agreement with Pakistan to build a 1000 Megawatt(MW) plants amid allegations that the deal violated NSG norms.
Both China and Pakistan have been routinely denying that the deal violates NSG norms, saying that it is for peaceful purposes and within the safeguards of nuclear watchdog IAEA.
The deal to build powerful reactors came after the successful conclusion of the Indo-US nuclear deal enabling India to build several nuclear plants and access key civil nuclear technologies and fuel.
China waited till the last minute before granting a crucial waiver enabling India to carry out nuclear commerce, when it came up for NSG approval in September 2008. Beijing raised some questions, but later supported it along with other members.
Pakistan, a close ally of China, is also demanding a similar waiver. Islamabad, like New Delhi, is not a signatory of the Nuclear Non-Proliferation Treaty, an important precursor to NSG membership.