A "small specialist team" of 12 UK soldiers are training Kurdish forces fighting Islamic State militants in Iraq, the Ministry of Defence has said.
It said troops from the Yorkshire Regiment are training Iraqi Kurds to use UK-supplied heavy machine guns.
The UK has already sent supplies, including weapons, to northern Iraq to help Kurdish Peshmerga forces.
Royal Air Force Tornado jets based in Cyprus have been flying combat missions over Iraq since September.
The UK training mission comes amid heavy fighting between Syrian Kurds and IS forces in the town of Kobane, in Syria.
Last month, the Ministry of Defence (MoD) announced it was supplying Kurdish forces, known as the Peshmerga, with 40 heavy machine guns, as well as thousands of rounds of ammunition.
The RAF has also transported military equipment and ammunition to northern Iraq on behalf of other countries, while providing equipment such as body armour, helmets and ration packs.
'Non-combat army trainers'
Confirming the deployment, an MoD spokesman said the government had previously made clear its intention to provide training to the Peshmerga as part of a "continued effort" to assist the fight against IS.
"The defence secretary has approved the deployment of a small specialist team of non-combat Army trainers, which is now in the Irbil area providing instruction on operating, employing and maintaining the heavy machine guns that were gifted by the UK last month," the spokesman added.
The last UK combat troops left Iraq in April 2009, with a small number staying on to train Iraqi forces until 2011.
BBC defence correspondent Jonathan Beale said the team of 12 UK soldiers, from 2nd Battalion the Yorkshire Regiment, is expected to spend a week in Irbil, in Kurdish-controlled northern Iraq.
"The MoD insists the soldiers will not be in a combat role - they're few in number and will be on the ground for just a week," our correspondent said.
"But there is a promise that Britain will be doing more to help train Iraq's security forces in the coming weeks and months. But so far, Britain's contribution to this US led effort has been modest."
Air strikes
RAF Tornados have been flying combat missions over Iraq since late September, following a vote in Parliament, while aircraft have been carrying out surveillance missions since the summer.
The UK is among more than 40 nations that have joined forces to challenge the IS militant group, which has seized large parts of Iraq and Syria.
Countries including the US have also taken part in air strikes against militant positions in Syria, but the UK military effort has so far been confined to Iraq.
The decision to send troops to train Peshmerga forces comes amid heavy fighting for the strategic border town of Kobane, on the border between Syria and Turkey.
At least 553 people are reported to have died in a month of fighting for the town, including 298 IS fighters, the Syrian Observatory for Human Rights said.
US aircraft have bombed IS positions as Kurdish fighters defend the town's border crossing with Turkey, but Kurdish leaders have said they remain outgunned on the ground.
Kobane official Idris Nassan told Reuters news agency that "the supply of fighters is very good".
"But fighters coming without arms, without weaponry, is not going to make a critical difference," he added.
(BBC)