North Korea's trade with China gained 4.4 percent from a year ago in the first nine months of this year, new data showed Tuesday, raising questions about the effectiveness of sanctions put in place to punish the North for conducting its third nuclear test earlier this year.

Trade volume rose to US$4.69 billion between January and September from $4.49 billion for the same period last year, according to the data released by the China Customs Information Center.

The data, seen by Yonhap News Agency, showed that North Korea's exports to China jumped 9.4 percent to $2.09 billion during the nine-month period, while its imports from China fell 2.3 percent to $2.6 billion.

A South Korean diplomatic source in Beijing suspected that North Korea's shortage of hard currency might be a factor for the decline in imports.

"North Korea's lack of foreign currency may be partly attributable to the fall in imports of Chinese goods," the source said on the condition of anonymity.

During the first nine months of this year, North Korea's imports of Chinese crude oil, however, rose to 415,000 tons, compared with 402,000 tons for the same period last year.

China did not export crude oil to North Korea in June and July this year, but resumed crude exports in August, according to the source.

In August and September, China exported 165,000 tons of crude oil to North Korea, the source said.

China, North Korea's most important ally and economic benefactor, voted for tightening U.N. sanctions against the North after Pyongyang conducted its third and most powerful atomic test in February.

Since then, Beijing has stepped up its diplomatic efforts to resume the long-stalled talks aimed at ending the North's nuclear weapons program. The six-party talks, involving the two Koreas, the U.S., China, Russia and Japan, have been dormant since late 2008.

China's chief nuclear envoy Wu Dawei arrived in Pyongyang on Monday, days after visiting Washington for talks with U.S. counterparts, including Glyn Davies. (Yonhap News)