By Joe Parkinson

ISTANBUL—Kurds in Iraq and Syria set aside long-held rivalries and took steps to unify their forces this week to battle Islamic State, gaining greater international legitimacy but magnifying fears in Turkey that a powerful enemy is on the rise.

The Kurdistan regional government in Iraq approved on Wednesday the deployment of 150 soldiers equipped with heavy weapons to relieve fellow Kurds in the besieged Syrian city of Kobani. Turkey, under U.S. pressure, agreed to allow those reinforcements to transit its territory despite concerns that this could indirectly strengthen a Kurdish militant group in Turkey that has fought the state for decades.

The U.S.-led coalition against Islamic State, also known as ISIS or ISIL, is increasingly relying on Kurdish ground forces. This is also a concern for the Iraqi government in Baghdad, which worries Kurds there will assert demands for independence more forcefully.

“Kurdistan has emerged as the most reliable partner in the international community in the war against ISIS,” said Barham Salih, former prime minister of the Kurdistan Regional Government in Iraq. “We have different views but we have a resourceful enemy at the gate and now there is a consensus emerging.”

The deal to deploy Iraqi Kurdish forces known as Peshmerga capped a weeklong meeting of regional Kurdish factions in the city of Dohuk in Iraqi Kurdistan. The meeting yielded a slate of power-sharing agreements between the Syrian Kurds leading the fight for Kobani and smaller Syrian parties closer to the president of the Kurdish regional government in Iraq, Massoud Barzani.

Mr. Barzani is an ally of Turkey, which supports the deployment of Peshmerga forces. But Ankara sees the Syrian Kurdish militia in Kobani as an enemy because of its close links with the Kurdistan Workers’ Party, or PKK, a group the U.S. and Turkey both list as a terrorist organization.

“The Peshmerga will arrive in Kobani within days,” said Fuad Hussein, chief adviser to Mr. Barzani. “We still have many differences but we are fighting Islamic State together across this territory.”

The pacts came after the outgunned Syrian Kurdish militia in Kobani—including avowedly secular and female fighting units—won international acclaim for repelling an assault by Islamic State forces that began more than a month ago. On Monday, U.S. planes dropped weapons and ammunition to the Syrian Kurds fighting in Kobani for the first time, despite Turkey’s objections.

The prospect of greater Kurdish political and military collaboration has prompted anxiety and debate in Turkey. The government there is struggling to juggle an increasingly complex Kurdish policy while maintaining peace talks with its own Kurdish minority after three decades of conflict with the PKK that have left some 40,000 dead.

The battle for Kobani has infected Turkish politics, sparking protests and clashes in which 40 Kurds died and threatening to derail the peace process.

Turkey’s estimated 15 million Kurds are bitter over the government’s failure to aid the city while many Turks demand tougher action to stem the rise of Syrian Kurds linked to the PKK.

The battlefield rise of the Syrian Kurdish fighters has also aggravated strains between Turkey and the U.S. over Syria policy, as Washington led more than 100 airstrikes to save Kobani and coordinated increasingly closely with the militia in secret. The U.S. has been pushing for greater Kurdish unity and has been pleased with the progress in that direction.

“Turkey is worried that the Syrian Kurds are emerging as heroes and wants to contain that by backing greater influence for their Iraqi Kurdish allies,” said Henri Barkey, professor of international relations at Lehigh University. “Until now, the U.S. deferred to the Turks on Kurdish issues. But on the Syrian Kurds, they didn’t and Ankara is reeling.”

The strains were on display Wednesday when Turkish President Recep Tayyip Erdogan said it was wrong for the U.S. to have dropped weapons to the Syrian Kurds and accused the U.S. of inadvertently allowing some of it to fall into the hands of Islamic State.

The Pentagon said one of the bundles of small arms, ammunition and supplies that was dropped drifted off course and was grabbed by Islamic State fighters. The Defense Department made the admission after the extremist group released a video on Tuesday showing off hand grenades, medical aid and other supplies it said it retrieved from the drop.

“It has become clear that this was wrong,” Mr. Erdogan said of the airdrop.

One reason for Ankara’s sensitivity is that its peace process with the PKK is at a critical phase. The talks appeared to get a boost on Wednesday when jailed Kurdish leader Abdullah Ocalan said they were entering a final phase, while Prime Minister Ahmet Davutoglu‎said the process could be concluded “within the next few months.”

The fight for Kobani has emerged a symbolic test of the U.S.-led coalition’s will to defeat Islamic State and marks Iraqi Kurdish forces’ first foray into Syria’s war. The shift spotlights the growing sway of Kurdish fighters across the region, with Washington now relying on two separate, stateless Kurdish groups in Syria and Iraq as ground forces to back up its air campaign against the jihadists.

Kurds battling Islamic State have won support in Western capitals for their avowedly secular and pro-Western stance and for demonstrating impressive battlefield resolve.

“The Kurds have struck more deals than anyone expected because they are galvanized by a common threats from ISIS,” said Wladimir Van Wilgenburg, analyst at the Jamestown Foundation who attended the meeting in Dohuk. “They now have a model for collaboration across borders, even though it is unclear whether it will hold and what Turkey’s reaction will be.”

In Kobani on Wednesday, Idris Nassan, a local Kurdish official, said the city’s defenders were eagerly awaiting reinforcements as battles raged on.

Iraqi Kurdish officials said Wednesday that final details of the Peshmerga deployment were still being hammered out but the fighters would likely travel overland through Turkey in a convoy carrying artillery, submachine guns and other heavy arms.

Iraqi Kurdish official Hemin Hawrami said the heavy weapons would help the besieged fighters, who say they need armor-piercing weapons to fight the better-armed Islamic State militants.

Michael Knights, an Iraq and Syria analyst at the Washington Institute, a think tank, said the Peshmerga contingent of 150 men would be a powerful political symbol of unity.

“This is a force that could help the Kurds mount attacks and will give city’s defenders more confidence. But 150 Peshmerga doesn’t tend to swing battles against ISIS in Iraq,” he said.

WSJ