Venezuela's ambassador to the United Nations, Rafael Ramirez, on Friday slammed a US investigation into alleged corruption under his watch at state oil giant Petroleos de Venezuela (PDVSA) as "slander."

"Obviously it is slander, it is a mass of lies and manipulation," Ramirez told reporters at UN headquarters in New York.

"It is part of a campaign, a new phase against our country, our government and our revolution," he said.

Ramirez served as PDVSA president from 2004-2014, and was Venezuela's oil minister for 12 years during the same period. He served briefly as foreign minister before he was appointed to the UN job in 2014.

The Wall Street Journal reported on Thursday that US authorities have launched "a series of wide-ranging investigations into whether Venezuela's leaders used PDVSA to loot billions of dollars from the country through kickbacks and other schemes."

The paper quoted "people familiar with the matter" and said the probes were also attempting to determine whether PDVSA and its foreign accounts were used for other illegal purposes, including black-market currency schemes and laundering drug money.

"I am calm, I have a clear conscience," Ramirez told reporters on Friday outside the UN Security Council, on which Venezuela is currently a rotating member.

The US and Venezuela have not had ambassadors in their respective capitals since 2010.

The relationship deteriorated further after the White House adopted sanctions against some Venezuelan officials in March.