South Korea's Unification Ministry said on Thursday that there is no stance or specific comments to make about condolences sent a day earlier by the Democratic People's Republic of Korea (DPRK) to victims of the ferry sinking disaster.

"There is no specific comment to make about the condolences which North Korea (DPRK) sent," a ministry official said over phone. The official said the two Koreas exchanged such condolences before.

The DPRK sent its condolences on Wednesday via the fixed line at the truce village of Panmunjeom, saying that it conveys its condolences to the numerous dead or missing passengers, including young students, according to the ministry.

The 6,826-ton ferry Sewol capsized and sank off Jindo Island, a southwestern tip of South Korea, on April 16. The ninth day into search, 171 people have been confirmed dead, leaving 131 others still missing. The number of the rescued has been unchanged at 174 since the calamity occurred.

The DPRK's condolence could have had a significant meaning in the inter-Korean relations, possibly a conciliatory gesture, but the South made no favorable, nor antagonistic, response to it as possibilities were running high for Pyongyang's fresh nuclear test.

Defense Ministry spokesman Kim Min-seok told a press briefing on Tuesday that the DPRK stepped up activity in its main underground test site, saying increased activity was detected in and around the Punggye-ri test site.

Pyongyang staged its third nuclear test at the Punggye-ri site in February last year, two months after launching a three-stage rocket, called Unha-3, which Seoul has claimed was a long-range missile.

Kim declined to specify what activities are being made, but local media reported quoting military officials that the site has recently been cloaked by fence screens and many vehicles were coming in and out of the place.

He said the military was thinking of possibilities that the DPRK may carry out a nuclear test by surprise in a short period of time or pretends to conduct the test as seen in past cases.