The Democratic Republic of Congo on Wednesday said foreign "greed" for its cobalt lay behind the "malicious" pressures weighing on the country's electoral process.

The DRC is in the grip of a crisis over plans to hold presidential elections, which were delayed by President Joseph Kabila's refusal to step down on the expiry of his second and final term last December.

Under a compromise deal, the vote was scheduled to take place this year.

But on October 12, the country's electoral commission said logistical problems in violence-troubled regions made it impossible to hold the elections before early 2019.

After the United States demanded the vote be held next year, the commission last week said the election will take place on December 23, 2018.

In a press conference on Wednesday, government spokesman Lambert Mende condemned what he called "an avalanche of external pressures on (DRC's) electoral process."

The pressures, he charged, were dictated by "greed" abroad for DRC's "immense strategic natural resources".

"After rubber, copper and coltan respectively, it is now our reserves in cobalt which are the cause of these malicious intrusions," Mende said, without elaborating.

- UN peacekeeper dies -

A mineral-rich country a quarter the size of Europe, but mired in poverty and corruption as well as scarred by ethnic divisions and fighting in its east, DRC is one of Africa's most volatile nations.

Western countries, as well as the influential Roman Catholic church, have led the effort to prevent the crisis over Kabila's departure from erupting into bloody violence.

In a statement on Monday supporting the December 2018 timetable, the US State Department said the DRC had taken "a significant step toward realizing its first peaceful, democratic transfer of power".

But it also noted "the importance" of Kabila abiding by the constitution and pledges reached under a compromise agreement last year to step down following elections.

The 46-year-old has been president since 2001 after taking over from his assassinated father, Laurent.

Separately, the UN peacekeeping mission in Congo, known as MONUSCO, on Wednesday said it "regretted" that the elections were being postponed again, but offered its help for organising them.

It also announced that a Tanzanian UN peacekeeper had died from his wounds after his unit was attacked on October 9.

Two other UN troops also died and 18 were wounded in the attack, which occurred near Beni, in the troubled province of North Kivu.

The assault was attributed to Allied Democratic Forces (ADF), which is dominated by hardline Ugandan Muslims.

The ADF has been accused by Kinshasa and the UN mission of killing more than 700 people in the Beni region since October 2014.