The Niger Delta Avengers, a militant group that has been carrying out attacks on Nigerian oil facilities in the past few months, claimed responsibility on Sunday for five new attacks in the southern energy hub since Friday.
The group had previously not laid claim to any attacks in the Niger Delta - the source of most of the OPEC member's oil - since June 16.
Petroleum Ministry sources said in late June that a month-long truce had been agreed with militants. But the Avengers said they did not "remember" agreeing to a ceasefire.
Attacks in the Niger Delta have pushed Nigerian crude production to 30-year lows, although the Nigerian National Petroleum Corporation (NNPC) said last week that output was rising because of repairs and a fall-off in attacks.
In messages posted on Twitter in the early hours of Sunday, the Avengers said they had attacked a pipeline connected to the Warri refinery operated by NNPC on Friday night.
They added that they blew up two lines on Saturday night close to Batan flow station in Delta state run by NPDC, a subsidiary of NNPC.
The militants also said two Chevron facilities close to Abiteye flow station, in Delta state, came under attack early on Sunday.
Residents in some of those areas reported hearing blasts.
"All five operations" were carried out by an Avengers "strike team", the group said.
Garba Deen Muhammad, a spokesman for state oil company NNPC, whose managing director is the oil minister, confirmed that the crude facilities identified by the Avengers had been attacked.
"Government will not be deterred in its efforts to find a lasting solution to these attacks," he said.
Chevron spokeswoman Isabel Ordonez said that "as a matter of long-standing policy," the company did not comment on "the safety and security" of its personnel and operations.
The militants say they want a greater share of Nigeria's oil wealth, which accounts for around 70 percent of national income, to be passed on to communities in the impoverished region and for areas blighted by oil spills to be cleaned up.
On Thursday, President Muhammadu Buhari hosted a group of community leaders from the Delta and urged them to pacify people in the restive region where anger is widespread.
Eric Omare, of the Ijaw Youth Council (IYC), which represents the Delta's biggest ethnic group, said the "resumption" of attacks was "worrisome", adding that the government had failed to build on goodwill generated by the oil minister's visit to the region in June.
"The federal government has not taken any practical step towards resolving the issues," said Omare, adding that the IYC urged the Avengers not to carry out further attacks in order to "give room for constructive dialogue".