Asif Ali Zardari, the president of Pakistan, has concluded his fifth visit to China since he came to power in 2008.

Amid much mutual backslapping and loud calls from the Pakistani president for more Chinese investment in his country's ravenous energy sector, Zardari and Hu Jintao, his Chinese counterpart, have stayed almost silent on the biggest of their shared concerns.

Neither side was expected to trumpet their blockbuster civilian nuclear agreement, which could knock another hole in the developing world's non-proliferation regime and lead Islamabad farther down the road away from Washington and towards Beijing.

The deal for China to design, build and finance two new nuclear reactors at an estimated cost of nearly $2bn has been out in the open for more than a year, but it is technically forbidden under international rules.