* Villagers rounded up and shot outside state capital
* Incident comes a day after scores killed in three villages
* President under pressure over insurgency, mass abduction (Adds witness, background, detail)
By Lanre Ola
MAIDUGURI, Nigeria, June 5 (Reuters) - Suspected Islamist militants dressed as soldiers rounded up and killed at least 42 villagers in northeastern Nigeria, as an escalating insurgency increasingly targets civilians, a police source said.
The shootings on the outskirts of the city of Maiduguri late on Wednesday came a day after officials said raiders killed scores in three other settlements in Borno state, where Boko Haram insurgents first launched their campaign to carve out an Islamist caliphate.
Boko Haram has stepped up its revolt and mounted nearly daily attacks since it grabbed world headlines in April by abducting more than 200 schoolgirls in another part of the state.
The mass abduction, and Boko Haram's fight-back against a military offensive, has increased political pressure on President Goodluck Jonathan who has faced regular street protests by activists criticising his response.
Jonathan has accepted help from the United States and other foreign powers who are alarmed at the prospect of further turmoil in Africa's largest economy and oil producer, and its potential impact on a fragile region. Borno state borders Niger, Chad and Cameroon.
The gunmen in military uniform arrived in three vehicles, called the civilians together in the village of Bardari, then opened fire, the police source told Reuters. "The people couldn't identify them in time as terrorists."
The militants then left, crossing a river and setting fire to houses in the neighbouring village of Kayamla, added the source.
"Boko Haram wreaked havoc in the villages. They burned houses and killed people mercilessly after tricking the residents," said Saleh Mohammed, a member of Civilian JTF - one of a number of vigilante groups that have sprung up to try to fight back.
Those civilian groups face revenge attacks by Boko Haram, which had focused mostly on military and government targets in the early days of its revolt.
No group claimed responsibility for the latest attacks. Boko Haram has no direct line of communication with the Western press and its purported leader, Abubakar Shekau, only occasionally claims attacks through videos circulated to local journalists.
Jonathan and the army have said they are doing all they can to release the girls, but have warned any attempt to free them by force could put them at risk, while any deals or prisoner swaps could encourage more kidnappings.
Britain's Foreign Secretary William Hague will host a meeting of African and Western officials in London next week aimed at stepping up efforts to defeat Boko Haram, his office said on Thursday. (Writing by Andrew Heavens; editing by Andrew Roche)