In the course of a week, several setbacks for President Bashar Al-Assad have provided fresh reminders of the strains facing the Syrian army and its allies in the four-year-long struggle for Syria.

In the southwest at the border with Jordan, the capture of a crossing by rebels suggests new resolve among their Arab backers who want to see Assad gone and to check the growth of Iranian influence across the Middle East.

In the northwest, near the border with Turkey, the capture of an entire city by Islamist groups including al Qaeda's Syrian wing has triggered claims in Damascus that Ankara has boosted its support for the insurgency.

The Islamic State group, the single most powerful insurgent force in Syria, has meanwhile started to threaten parts of the Syrian state still run by Assad, creeping westwards out of its northern and eastern strongholds. This week it massacred 45 people in a village in a government-held area near Hama city.

It is also battling rival insurgents for a Palestinian refugee camp on the Damascus outskirts that would give it a foothold just a few kilometres from Assad's seat of power.

Assad has appeared comfortable and confident in the territory he governs stretching north from Damascus through Homs and Hama and up to the coast - an area where the bulk of the population still live.

Backed by Iran and Russia, he has shown no sign of yielding to the calls of Western and Arab states for him to step aside. Damascus has appeared buoyed by the US-led fight against Islamic State, a shared enemy, confident that Washington will eventually be forced to stage an about-face and engage Assad.

The United States in contrast has maintained its long-held position that Assad must go. US Secretary of State John Kerry said last month military pressure may be necessary "given President Assad's reluctance to negotiate seriously".

The Syrian air force still controls the skies, giving the army an important edge. Even as it loses ground in some places, the state reported gains this week against insurgents in an area of vital importance near the Lebanese border.

But in more remote areas, such as the northwestern city of Idlib that fell to insurgents on Saturday, and the Jordanian border crossing of Nasib in Deraa province that fell to rebels on Wednesday, it appears more difficult for Damascus to hold on.

"It is indicative of the manpower constraints, which are going to be a continuing and ever worsening problem for the regime going forward," said Noah Bonsey, a senior analyst at International Crisis Group.

"It's not that this threatens control of its core areas, from Damascus through Homs and up to the coast. But outside of those core areas, the regime is simply not able to replace lost manpower," he said.

The army continues to report operations in both Idlib and Deraa provinces. The state news agency said the military killed insurgents in both places on Wednesday.

JORDAN WANTS TO KEEP IRAN "OUT OF OUR BACKYARD"

The rebels in southern Syria, some of whom have received small amounts of military aid funnelled via Jordan, have been seen as one of the big remaining risks for Assad.

Their capture of the Nasib crossing signals a possible shift in Jordan's position towards the war. Jordan is an ally of the United States and conservative Sunni-led Gulf states including Saudi Arabia that want Assad gone. But it has plotted a careful course during the war, fearing a military backlash from Damascus.

Until Wednesday's rebel attack, Jordan had used its influence over the southern insurgents to prevent them from taking the crossing. But a Jordanian official said these calculations had changed with Assad appearing more reliant on Shi'ite Islamist Iran and Lebanon's Hezbollah. Both played central roles in an offensive launched in February by Damascus to reclaim the south.

"We cannot allow Iran to come to our backyard. They have entertained too many ideas lately with Soleimani calling the shots and made everyone feel we were beginning to see the rise of an Iranian empire," the official said.

He was referring to Major General Qassem Soleimani, an Iranian military commander who has been directing operations in Iraq to recapture the city of Tikrit from Islamic State and is seen by Tehran's Arab rivals as the symbol of its growing role.

The rebels say Iran was leading the offensive to reclaim the south - a view echoed by a senior Western diplomat following Syria. Iran's interests include establishing a foothold at the border with Israel.

But the offensive slowed after some early but limited gains. The rebels say their foreign backers responded by sending them extra military support, while complaining it remained inadequate.

Last week, the rebels also captured the historic town of Bosra al Sham, which they described as a garrison for pro-government fighters, including Hezbollah.

An insurgent leader in southern Syria said that by taking Nasib, the rebels were cutting the army's supply lines into the city of Deraa, suggesting this may be the next target.

"The liberation of Nasib is part of a strategy to completely liberate Deraa and is an essential step that was preceded by the liberation of Bosra only a few days ago," Saber Safar, a defected army colonel, told Reuters.

LOSING THE NORTHWEST

While the south marks the last significant foothold for the mainstream, non-jihadist rebels, the north has been largely taken over by Islamists of the type that seized the city of Idlib on Saturday.

The Syrian government says both Idlib and Bosra Al Sham were subjected to fierce attack by the Al Qaeda-linked Nusra Front.

Idlib was only the second provincial capital to slip from state control since the start of the war. The first was Raqqa, which Islamic State has turned into the de facto capital of its cross-border "caliphate".

The Islamist groups that took part included the hardline Ahrar al-Sham and the Al Qaeda-linked Nusra Front.

A Syrian military source earlier this week accused Turkey, which is hostile to Assad, of helping them, including through the provision of advanced communication equipment.

While not an area of strategic importance to Damascus, losing Idlib marked a symbolic blow. A second Western diplomat said the army was quick to withdraw forces in areas where it is under pressure so as to minimise casualties.

A US intelligence official said the forces that took Idlib included "moderates".

"Although moderates have faced some setbacks in the north, their position is stabilizing. They are right in the thick of the fighting, having joined with other opposition groups to seize Idlib from the regime," said the official. - Reuters