Maoist rebels in Nepal have called off a four-month unilateral truce.
The rebels have fought an armed insurgency for 10 years
Rebel leader Prachanda said the army, which has refused to halt military operations, was to blame for the decision to resume hostilities.
Hours after the announcement three bombs went off in the towns of Pokhara and Bhairahawa. No one was hurt.
There had been calls from political parties, rights groups and the UN for the rebels to extend the truce, which ended at midnight on Monday (1815GMT).
The truce saw a sharp drop in deaths, although both rebels and the army still committed rights abuses, monitors said.
The BBC's Sushil Sharma in Kathmandu says the rebel move has sparked fears of an upsurge in violence in the kingdom.
The rebels said they were "compelled to go on the offensive".
"The royal army is surrounding our people's liberation army, which is in defensive positions, to carry out ground as well as air attacks on us," a statement said.
Nepal's government said it was prepared for any eventuality and called the end of the ceasefire unfortunate.
Nepal's neighbour, India, and the United States both voiced concern at the rebel move.
Rebel leader Prachanda promised to disrupt next month's local elections, which opposition parties have pledged to boycott.
He also said an alliance between the rebels and opposition parties aimed at ending direct rule by the king would continue.
The government refused to match the rebel truce offer, saying they were not sincere. It has accused them of using previous ceasefires to rearm.
King Gyanendra, who seized absolute power in February, has promised to crush the rebels and says they must disarm before any peace talks.
Opposition parties have accused him of trying to provoke the rebels into returning to violence.
More than 12,000 people have died in the 10 years since the Maoists began their fight to replace the monarchy with a communist republic.