* Islamic State tightens control in tribal region
* Other rebels, tribes attempt to fight the group
* Islamic State executes 25 in Deir al-Zor province (Adds quote from Deir al-Zor resident, changes byline)
By Oliver Holmes and Suleiman Al-Khalidi
BEIRUT/AMMAN, Aug 11 (Reuters) - Islamic State has crushed a pocket of resistance to its control in eastern Syria, crucifying two people and executing 23 others in the past five days, a monitoring group said on Monday.
The insurgents, who are also making rapid advances in Iraq, are tightening their grip in Syria, of which they now control roughly a third, mostly rural areas in the north and east.
Fighters from the al-Sheitaat tribe in eastern Deir al-Zor had tried to resist Islamic State's advance this month, according to residents near the area and the Syrian Observatory for Human Rights, a Britain-based monitoring organisation.
In al-Shaafa, a town on the banks of the Euphrates river, Islamic State beheaded two men from the al-Sheitaat clan on Sunday, the Observatory said, and gave residents a 12-hour deadline on Monday to hand over members of the tribe.
In other parts of Deir al-Zor province, the militants crucified two men for the crime of "dealing with apostates" in the city of Mayadin, and two others were beheaded for blasphemy in the nearby town of al-Bulel, the Observatory said.
Islamic State, which has fought the Syrian army, Kurdish militias and Sunni Muslim tribal forces, has made rapid gains in Syria since it seized northern Iraq's largest city, Mosul, on June 10, and declared an Islamic caliphate.
The Observatory said a further 19 men from the al-Sheitaat tribe were executed on Thursday, 18 shot dead and one beheaded, on the outskirts of Deir al-Zor city. It said the men worked at an oil installation.
"No one will now dare from the other tribes to move against Islamic State after the defeat of the al-Sheitaat," said Ahmad Ziyada al-Qaissi, an Islamic State sympathiser contacted by Skype from Mayadin.
Tribal sources say the conflict between Islamic State and the al-Sheitaat tribe, who number about 70,000, flared after Islamic State took over of two oil fields in July.
One of those, al-Omar, is the biggest oil and gas field in Deir al-Zor and has been a lucrative source of funds for rebel groups.
The head of the al-Sheitaat tribe, Sheikh Rafaa Aakla al-Raju, called in a video message for other tribes to join the fight against Islamic State.
"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 targeted after al-Sheitaat. They are the next target," he said in the video, posted on YouTube on Sunday.
A Syrian human rights activist from Deir al-Zor who fled for Turkey last year said rebels opposed to President Bashar al-Assad had retreated to al-Sheitaat tribal areas from which they had been trying to mount resistance to Islamic State.
He said, on condition of anonymity, that the resistance had been crushed in the last few days. "The situation is very bad, but the people can't repel them," he said.
He said that, in tandem with their violent campaign, Islamic State was distributing gas, electricity, fuel and food to garner local support.
"It is a poor area. They are winning support this way. They won a lot of support this way. They are halting theft and punishing thieves. This is also giving them credibility."
Another resident of Deir al-Zor, Abdullah al-Noami, said that four al-Sheitaat towns had fallen.
"These areas have fallen into the hands of Islamic State after the withdrawal of the (al-Sheitaat) fighters. The youths who were found were executed or their heads were cut off on the grounds that they fought against Islamic State," he said.
More than 170,000 people have been killed in Syria's civil war, which pits overwhelmingly Sunni Muslim rebels against 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, with Islamic State emerging as the most powerful.
In Raqqa, Islamic State's power base in Syria, its hold appears to be growing only firmer even as Syrian government forces intensify air strikes on territory held by the group.
One Syrian living in an area of Islamic State control near Raqqa said the number of its fighters in the streets had grown dramatically in the last few weeks, particularly since it captured the army's 17th Division at the end of July.
The group has levied a tax on non-Muslims, and settled foreign fighters in confiscated homes, said the resident, who asked for anonymity due to security concerns.
But despite that, as in Deir al-Zor, it has won a degree of respect among locals by curbing crime using their version law of and order. For youths without work, salaries offered by Islamic State are one of the few sources of income.
"The (Islamic) State has respect and standing and its voice is heard," said the resident, speaking by Skype. (Additional reporting by Tom Perry in Beirut; Editing by Robin Pomeroy)