Will the thousands of foreign troops in Syria ever leave?

Syrian regime President Bashar al-Assad has regained control over large swathes of Syria with critical help from Iran and Russia, appearing to put to rest the possibility of a military overthrow.

But his rule is reliant on continued assistance from Iranian-sponsored militia groups which have spread across the war-torn country.

The international fight against the Islamic State group has also provided a justification for foreign forces to be deployed in Syria, but foreign powers with longer-term ambitions are looking to maintain a presence in the country for years to come.

Some countries have already indicated they plan to stay in Syria for the foreseeable future. So who is fighting in Syria, and how long will they stay?

United States

US forces were initially deployed in northern Syria to help train and support Kurdish-dominated forces fight the Islamic State group, but their number has gradually grown.

The official limit on US forces remained at 503 since shortly before President Barack Obama left office but the actual number now is believed to be more than 1,500.

Special Forces, a Marine artillery unit, forward air controllers and others are spread across more than a dozen bases in norther Syria.

The end of the fight against IS would take away any legal justification for the presence of US troops but US officials have suggested they plan to maintain a presence in the north until an overall settlement is found for the conflict.

Kurdish officials have asked the US to stay on, fearing that a quick withdrawal would facilitate Assad’s forces swooping in on Kurdish-held territory in the north.

Earlier this month, the Syrian government called on the United States to withdraw its forces now that the fight against the Islamic State group is nearly over.

Russia

Russia has never said officially how many of its military personnel, warplanes or other weapons are in Syria, but voter turnout figures from abroad in September 2016 parliamentary elections put the number at around 4,300.

The Russian presence has likely increased, as Moscow this year deployed its military police to patrol so-called "de-escalation zones" in Syria.

Open-source materials – including video from the Hemeimeem air base, the main hub for the Russian military in Syria since its campaign began in September 2015 – indicate that Russia has several dozen jets and helicopter gunships there.

Syria has allowed Russia to use Hemeimeem air base indefinitely without cost. Moscow also has signed a deal with Syria to use the Tartus base for 49 years, which could be extended if both parties agree

Russia also has deployed Special Forces to conduct intelligence and coordinate airstrikes. Senior Russian military officers also have helped train and direct Syrian government troops. In recent months, Russian military police have become increasingly visible in Syria.

The chief of the Russian military general staff, General Valery Gerasimov, said last week that Russia will "significantly" reduce its military foothold in Syria as the campaign nears its end.

At the same time, he indicated Russia will maintain a presence at both the Hemeimeem air base and the navy supply facility in Tartus.

Gerasimov added that the military's Reconciliation Centre, a group of officers who have helped negotiate and maintain truces in Syria and coordinated the delivery of humanitarian aid, also will stay.

Syria has allowed Russia to use Hemeimeem air base indefinitely without cost. Moscow also has signed a deal with Syria to use the Tartus base for 49 years, which could be extended if both parties agree.

The Russian military plans to modernise the air base to allow it to host more warplanes. It also intends to expand the Tartus facility significantly to make it a full-scale naval base capable of hosting warships, including cruiser-sized vessels.

Iran and its militias

Iran has made an enormous effort to keep Assad in power and has provided extensive military and financial support throughout the six-year war.

Tens of thousands of Iranian-sponsored pro-government local militias known as the National Defence Forces are deployed across Syria, in addition to Iraqi Shia militias and thousands of Iranian-backed Hizballah fighters from Lebanon who have been key factors in turning the war in the government's favour.

Iran has also deployed the Islamic Revolutionary Guards Corps in Syria as well as Iranian officers who provide military and political support.

Iran has made an enormous effort to keep Assad in power and has provided extensive military and financial support throughout the six-year war

Officials say at least 1,000 Iranian fighters have been killed in Syria and Iraq.

Hizballah has deployed in areas along Syria's border with Lebanon where the group has built military facilities and long-term bases it is unlikely to leave soon.

Iran's strategy aims to ensure it can continue to pursue its vital interests after the war, using parts of Syria as a base.

Turkey

Turkey sent ground forces into Syria last year in a campaign called 'Operation Euphrates Shield' against the Islamic State group and Syrian Kurds along its border with Syria.

Turkish officials have not disclosed how many Turkish soldiers are deployed in Syria but security experts estimate that at least 2,500 troops are stationed in a swathe of territory revolving around the towns of al-Rai, al-Bab and Jarablus – a border zone that Turkey and Turkey-backed rebels took back from IS last year under "Euphrates Shield".

An estimated 400 more Turkish troops are in the Idlib region as part of an agreement reached among Turkey, Russia and Iran to create a "de-escalation zone" in the area.

Turkey is building schools and hospitals in areas liberated under "Euphrates Shield" to encourage the return of refugees, and it was unclear how long the Turkish troops would stay in the zone.

President Recep Tayyip Erdogan has suggested that the Turkish troops could target a Syrian Kurdish group that Turkey considers to be a security threat in the Afrin region, north of Idlib, once the "de-escalation" mission is over.