By Omer Berberoglu

MURSITPINAR Turkey (Reuters) - Iraqi Kurdish peshmerga fighters and moderate Syrian rebels bombarded Islamic State positions in Kobani on Monday, but it was unclear if their arrival would turn the tide in the battle for the besieged Syrian border town.

Kobani has become a symbolic test of the U.S.-led coalition's ability to halt the advance of Islamic State, which has poured weapons and fighters into its assault of the town that has lasted more than a month.

The battle has deflected attention from significant gains elsewhere in Syria by Islamic State, which has seized two gas fields within a week from President Bashar al-Assad's forces in the center of the country.

In Iraq, the group has executed more than 300 members of a Sunni tribe that dared oppose it last week, after seizing the tribe's village in the Euphrates valley west of Baghdad. On Monday a member of the tribe said another 36 members had been executed in the provincial capital Anbar.

For now, the eyes of the world have been on Kobani, where weeks of fighting have taken place within full view of the Turkish border, causing outrage among Kurds in Turkey who blamed their government for doing too little to help defend the town.

The arrival in Kobani of the Iraqi Kurdish peshmerga and additional Syrian Free Syrian Army (FSA) fighters in recent days has escalated efforts to defend the town after weeks of U.S.-led air strikes slowed but did not reverse the Islamists' advance.

White smoke billowed into the sky as peshmerga and FSA fighters appeared to combine forces, raining cannon and mortar fire down on Islamic State positions to the west of Kobani, a Reuters witness said.

The U.S. military said it bombed Islamic State positions in Syria five times and in Iraq nine times on Sunday and Monday, including near Kobani.

An estimated 150 Iraqi Kurdish fighters crossed into Kobani with arms and ammunition from Turkey late on Friday, the first time Ankara has allowed reinforcements to reach the town.

Story continues

"(Their) heavy weapons have been a key reinforcement for us. At the moment they're mostly fighting on the western front, there's also FSA there too," said Meryem Kobane, a commander with the YPG, the main Syrian Kurdish armed group in Kobani.

She said fierce fighting was also continuing in eastern and southern parts of the city.

The peshmerga, the official security forces of Iraq's autonomous Kurdish region, have deployed behind Syrian Kurdish forces and are supporting them with artillery and mortar fire, according to Ersin Caksu, a journalist inside Kobani. The fiercest fighting was taking place in the south and east, areas where the reinforcements were not deployed, he said.

Despite weeks of air strikes, Islamic State has continued to inflict heavy losses on Kobani's defenders. Late last week hospital sources in Turkey reported a jump in the number of dead and wounded Kurdish fighters being brought across the frontier.

TRIBES

In Iraq, Islamic State fighters have stormed through mainly Sunni Muslim cities and towns in the Tigris and Euphrates valleys north and west of Baghdad, in part with the support of many Sunni Muslims angry at perceived mistreatment by the Shi'ite-led government in Baghdad.

Washington hopes that Sunni tribes can be lured to switch sides, as they did during the U.S. "surge" campaign against al Qaeda in 2006-2007. But so far, Sunni tribes that have dared to stand up to Islamic State have suffered brutal fates, while complaining of little support from the Baghdad government.

More than 320 members of the Albu Nimr tribe, including women and children, have been hunted down, captured, shot and buried in mass graves since their village fell to the fighters.

Hamdan al-Nimrawi said on Monday 36 more members of the tribe had been shot dead in Ramadi, capital of the vast Anbar province west of Baghdad, where fighters control towns and villages stretching from the Syrian frontier, down the Euphrates to the western outskirts of Baghdad itself.

Setting up an international coalition to fight Islamic State in both Iraq and Syria has been a tricky diplomatic task for the United States, requiring consensus for intervention in two complex, multi-sided civil wars where nearly every country in the Middle East has a stake.

"PSYCHOLOGICAL WAR"

The fight for Kobani within sight of the Turkish frontier has heaped pressure on Ankara, which has been reluctant to intervene, accusing the town's defenders of links with Kurdistan Workers' Party (PKK) militants, who have fought a decades-long insurgency against the Turkish state.

Some 40 people died in riots in Turkey last month after Kurds, who make up around 15 percent of the population and the majority in the southeast, rose up in anger at the government for doing too little to help protect Kobani.

President Tayyip Erdogan on Monday decried what he called a "psychological war" being waged by international media against Ankara over its Syria policy.

A survey by pollster Metropoll appeared to show sympathy for Erdogan's stance, with a majority of respondents saying the PKK, listed as a terrorist organization by Europe and the United States, posed a greater threat to Turkey than Islamic State.

Three soldiers were killed last week by suspected Kurdish militants while out shopping, the latest attack on Turkish security forces amid growing tension over a stalled Kurdish peace process.

With the world's attention on Kobani, Islamist forces have continued to gain ground elsewhere in Syria.

The Islamic State seized a gas field in the central province of Homs, according to the SITE jihadist website monitoring service -- the second gas field reported captured in a week from Assad's forces.

On Monday, the British-based Syrian Observatory for Human Rights said a Western-backed Syrian opposition group, the Hazzm movement, had lost positions and equipment including heavy weapons after being overrun by al Qaeda-affiliated Nusra Front fighters in Idlib province, near the Turkish border.

On Saturday, Nusra fighters seized the bastion of another western-backed group, also in Idlib.

(Additional reporting by Michael Georgy in Baghdad, Humeyra Pamuk in Istanbul, Jonny Hogg in Ankara, Alexander Dziadosz in Beirut and Susan Heavey in Washington; Writing by Jonny Hogg and Peter Graff; Editing by Nick Tattersall, Dominic Evans and Crispian Balmer)