* Activist says 300 killed in one day
* Other tribesmen pledge allegiance to Islamic State (Adds quotes from activists and tribal statement, bullets)
By Oliver Holmes and Suleiman Al-Khalidi
BEIRUT/AMMAN, Aug 16 (Reuters) - The Islamic State militant group has executed 700 members of a tribe it has been battling in eastern Syria during the past two weeks, the majority of them civilians, a human rights monitoring group and activists said on Saturday.
The Syrian Observatory for Human Rights, which has tracked violence on all sides of the three-year-old conflict, said reliable sources reported beheadings were used to execute many of the al-Sheitaat tribe, which is from Deir al-Zor province.
The conflict between Islamic State and the al-Sheitaat tribe, who number about 70,000, flared after the militants took over two oil fields in July.
"Those who were executed are all al-Sheitaat," Observatory director Rami Abdelrahman said by telephone from Britain. "Some were arrested, judged and killed."
Reuters cannot independently verify reports from Syria due to security conditions and reporting restrictions.
Proclaiming a 'caliphate' straddling parts of Iraq and Syria, Islamic State has swept across northern Iraq in recent weeks, pushing back Kurdish regional forces and driving tens of thousands of Muslims, Christians and members of the Yazidi religious minority from their homes, prompting the first U.S. air strikes in Iraq since the withdrawal of American troops in 2011.
The insurgents are also tightening their grip in Syria, of which they now control roughly a third, mostly rural areas in the north and east.
An activist in Deir al-Zor who spoke on condition of anonymity told Reuters that 300 men were executed in one day in the town of Ghraneij, one of the three main towns of the al-Sheitaat tribal heartland, when Islamic State stormed the town earlier this week.
Another opposition activist from Deir al-Zor said residents of al-Sheitaat towns had been given three days to leave.
"Those who were executed during the storming of the al-Sheitaat area are around 300. The rest were killed in the course of the battles," he told Reuters on condition of anonymity to protect his identity.
Civilians fleeing al-Sheitaat towns had either taken sanctuary in other villages or travelled to Iraq, he said.
More than 170,000 people have been killed in Syria's civil war, which pits overwhelmingly Sunni Muslim rebels against President Bashar al-Assad, a member of the Shi'ite-derived Alawite minority, backed by Shi'ite militias from Iraq and Lebanon.
The insurgency is split between competing factions in Syria, with Islamic State emerging as the most powerful.
Tribal powers in Syria and Iraq have had to make the choice between fighting Islamic State or pledging allegiance.
On Friday, a video posted on YouTube showed men who said they were from the al-Sheitaat towns of Kishkeih and Abu Hammam pledging full support for Islamic State.
"We say that what Islamic State stands for is justice," a tribal member sitting in a room with dozens of other men said in a statement that was read out.
The head of the al-Sheitaat tribe, Sheikh Rafaa Aakla al-Raju, called in a video message for other tribes to join them in the fight against the militants.
"We appeal to the other tribes to stand by us because it will be their turn next ... If (Islamic State) are done with us the other tribes will be targeted after al-Sheitaat. They are the next target," he said in the video, posted on YouTube.
Islamic State was condemned on Friday in a U.N. Security Council resolution for "gross, systematic and widespread abuses of human rights and violations of international humanitarian law." (Editing by Stephen Powell and Sonya Hepinstall)