China offered on Wednesday to help Venezuela restore its power grid, after President Nicolas Maduro accused US counterpart Donald Trump of cyber "sabotage" that plunged the South American country into its worst blackout on record.

Maduro, who retains control of the military and other state institutions as well as the backing of Russia and China, has blamed Washington for his nation's economic turmoil and denounced opposition leader Juan Guaido as a puppet of the United States.

With the power blackout in its sixth day, hospitals struggled to keep equipment running, food rotted in the tropical heat and exports from the country's main oil terminal were shut down.

Speaking in Beijing, Chinese Foreign Ministry Spokesman Lu Kang said China had noted reports that the power grid had gone down due to a hacking attack.

"China is deeply concerned about this," Lu said.

"China hopes that the Venezuelan side can discover the reason for this issue as soon as possible and resume normal power supply and social order. China is willing to provide help and technical support to restore Venezuela's power grid."

He gave no details.

Power returned to many parts of the country on Tuesday, including some areas that had not had electricity since last Thursday, according to witnesses and social media.

But power was still out in parts of the capital of Caracas and the western region near the border with Colombia.

Information Minister Jorge Rodriguez said power had been restored in the "vast majority" of the country.

The blackout was likely caused by a technical problem with transmission lines linking the Guri hydroelectric plant in southeastern Venezuela to the national power grid, experts have told Reuters.

Maduro has blamed Washington for organising what he said was a sophisticated cyber attack on Venezuela's hydroelectric power operations.