Prachanda, the rebel leader or the Fierce One, said in a statement on Monday: "The four-month ceasefire which we extended has come to an end,
"We want to make it clear our future actions will be targeted against the autocratic regime. We are compelled to go offensive not only for the sake of peace and democracy but for the sake of self-defence."
There was no immediate comment from the government over the Maoist announcement.
The rebels "respected and deeply understood" appeals from the United Nations and the European Union to extend the truce and urged them to understand the compulsion of the rebels to abandon it, he added.
Earlier on Monday, rights groups and political parties urged the rebels to extend their unilateral ceasefire despite the government's refusal to match the pledge.
Sudip Pathak, a member of the National Human Rights Commission, said: "We have established contact with the rebel leadership and have been urging them to extend the ceasefire and allow peace to continue."
Under the ceasefire, the rebels pledged not to attack military or civilian targets, hoping to revive peace talks, but said they would continue to defend their positions.
"The four-month ceasefire which we extended has come to an end"
Prachanda, leader of Nepal's Maoist rebels
They have, however, continued to block highways, extort money and kidnap villagers for indoctrination sessions.
Ram Chandra Poudel of the Nepali Congress, the largest party in Nepal, said: "We have asked the rebels not to resume violence."
The country's Maoist rebels announced a three-month halt in attacks on 3 September and later extended it for one month.
The government has refused to join the ceasefire, calling it a ploy by the militants to buy time to re-organise.
The rebels, who claim to be inspired by Chinese communist revolutionary Mao Zedong, have been fighting to topple Nepal's monarchy and establish a communist state.
More than 12,000 people have died in the decade-long insurgency.