North Korea may hold its party congress in May without foreign guests as it faces toughened international sanctions, a source familiar with the isolated country said Tuesday.

"So far, there have been no noticeable signs that North Korea is engaged in 'invitation diplomacy' for the party congress," the source said.

It added that it is attributable to a fall in North Korea's clout under the sanctions regime following its fourth nuclear test in January.

The Workers' Party of Korea is preparing to hold the first party congress in more than three decades in early May, which will likely highlight Kim's one-man leadership.

The seventh party congress comes as the U.N. Security Council slapped the toughest sanctions to date punishing the North for its nuclear test and long-range rocket launch earlier this year.

A total of 177 delegates from 118 countries including China and Russia attended the sixth party congress held in 1980, according to Seoul's unification ministry. At the fifth congress in 1970, there were no foreign guests.

The upcoming congress will likely serve as a key event that will reveal the North Korean leader's policy direction in his fifth year in power. Kim inherited power following the sudden death of his father Kim Jong-il in late 2011.

The source said that North Korea is likely to present its vision for the future to help the leader consolidate his power.

"North Korea will likely trumpet what it would call a success of the congress, but there is a low chance that tangible accomplishments will come out under the sanctions regime," it added.

An official at the ministry said that the party congress will be held at least for two days with a chance that it would start on May 7. Pyongyang has yet to publicly announce the date for the event.

"There is no indication that massive events for the congress are in the offing," he said, adding that Pyongyang's preparation for the convention appears to be lackluster.

South Korea said that North Korea is expected to face squeezes in funding the party event as its dollar earnings have taken a hit under the tougher sanctions.

North Korea has mobilized ordinary citizens to prepare for it under the "70-day campaign of loyalty" and forced them to offer money to authorities, inviting growing discomfort from its people, it added.

Seoul said that if the sanctions are put in place for a long time, the North's economy is not likely to recover due to a shortage of dollars and products.

The official added that Kim's title could be upgraded as one of the main political decisions at the congress, which also includes a possible overhaul of the WPK's organizations.

South Korea is keeping close tabs on a possibility that North Korea may carry out its fifth nuclear test and launch a long-range missile ahead of the party event. (Yonhap)