* Sharif cancels trip to Davos, condemns attack
* At least 20 soldiers killed in bomb blast
* Taliban claim responsibility, vow more attacks (Writes through with PM, Taliban comments)
By Saud Mehsud and Haji Mujtaba
DERA ISMAIL KHAN/MIRANSHAH, Pakistan, Jan 19 (Reuters) - A bomb planted by Taliban insurgents ripped through a vehicle carrying Pakistani troops on Sunday, killing 20 soldiers and prompting Prime Minister Nawaz Sharif to cancel his trip to the World Economic Forum in the Swiss resort of Davos this week.
At least 30 others were wounded as the convoy prepared to leave the volatile northwestern town of Bannu for nearby North Waziristan, a lawless, tribal region on the Afghan border where many al Qaeda-linked militant groups are holed up.
The bold, daylight attack - the biggest on Pakistani security forces in months - dealt a major blow to the army at a time when Pakistan is already under strong U.S. pressure to do more to contain the insurgency on its western frontier.
The army said the bomb had been planted in a civilian vehicle rented locally in order to transport troops to North Waziristan. The device exploded as soldiers got inside the car and prepared to leave.
"With the help of God we claim responsibility for this," Pakistani Taliban spokesman Shahidullah Shahid said in a telephone call from an undisclosed location.
"The army is our enemy. We will carry out many more attacks like this."
The blast, which was heard all around Bannu, came despite recent hints by the Taliban that they might be theoretically open to the idea of peace talks with the government.
And yet attacks continue unabated, a concern to regional powers already anxious about security as most foreign troops prepare to leave neighbouring Afghanistan this year.
The group had earlier vowed to step up attacks on security and government forces after electing a new hardline leader, Mullah Fazlullah, at the end of last year.
A Pakistani police officer said the blast took place as a convoy of army troops and paramilitary Frontier Corps was preparing to leave Bannu, adding that security forces had cordoned off the area to investigate.
PM CANCELS DAVOS TRIP
Sharif condemned the attack and cancelled his trip to the Davos gathering which runs from Jan. 22 to 25 and draws thousands of the world's most influential people.
"Our nation is united against extremism and terrorism and the sacrifices rendered by our citizens and personnel of law enforcing agencies will not go in vain," he said in a statement.
Sharif came to power last year promising to step up efforts to engage the Taliban in peace talks and find a negotiated solution to years of fighting.
Even though both sides have made tentative overtures towards negotiations, no meaningful discussions have taken place for years. The Pakistani Taliban, who are loosely aligned with their Afghan namesakes, are also deeply fractured, making policy coordination all the more difficult within the group.
"The fact is that we have neither refused dignified, serious and meaningful talks in the past, nor will we deny the importance of negotiations in the future," Azam Tariq, a Pakistani Taliban spokesman, told Reuters Television on Friday.
"However, let me say that the government has never made any serious effort to hold talks."
Shadid, the other spokesman, denied the movement had decided to ditch talks altogether after the death of its previous leader, Hakimullah Mehsud, in a November drone strike which enraged the Taliban and hardened its stance against the government.
"There is a war going on between us and them, and in a war people get killed. They kill us, we kill them," he said. "But, even now, if the government can prove its sincerity and its authority, we are ready for talks." (Writing by Maria Golovnina; Additional reporting by Jibran Ahmad in Peshawar and Sheree Sardar in Islamabad; Editing by Simon Cameron-Moore and Mike Collett-White)