British liaison staff are embedded with US forces in the Horn of Africa, the Ministry of Defence has revealed, as concern grows about redeployment of the UK squadron of 10 armed Reaper drones.

Although three British officers are based at Camp Lemonnier in Djibouti – the US base from which unmanned strikes are launched against al-Qaida groups in Yemen – the MoD denies they are involved in co-ordinating such attacks.

Both the human rights organisation Reprieve and the Labour former defence minister Tom Watson have expressed anxiety over British involvement in covert drone operations beyond Afghanistan.

The MoD is thought to be reluctant to bring home its squadron of Reapers, controlled remotely by satellite from RAF Waddington in Lincolnshire, once operations in Afghanistan end later this year. Remotely Piloted Aircraft Systems (RPAS), as the RAF prefers to describe them, cannot fly freely in UK airspace due to Civil Aviation Authority restrictions.

The presence of British officers in Djibouti will heighten expectations that the ground is being prepared for redeployment of UK drones elsewhere overseas.

Ben Emmerson QC, the UN rapporteur on counter-terrorism monitoring operations by unmanned aircraft, has said that sources close to the MoD have told him the Reapers could be sent to Africa or the Middle East for future operations.

The presence of British troops in Camp Lemonnier was confirmed in a written parliamentary answer to Watson by the defence minister Mark Francois. He said: "[The three officers] work within the Combined Joint Task Force-Horn of Africa (CJTF-HOA) and are responsible for planning and supporting US military operations in the region. As embedded military personnel within a US headquarters they come under the command and control of the US armed forces, but remain subject to UK law, policy and military jurisdiction."

Watson, chair of the all-party parliamentary group on drones, has now written to Emmerson asking him to make sure the UK government "engages in informed debate on the imperative legal questions governing use of armed drones that you have identified" and responds to the rapporteur's latest report.

Watson's letter also warns that "parliamentary scrutiny of future basing options for UK armed drones may be avoided on the basis that use of remotely operated systems do not necessarily require physical deployment of UK troops because they can be operated from the UK".

It adds: "UK experts, including Professor Michael Clarke, director of the Royal United Services Club (RUSI) have indicated … that future basing options for UK Reapers include Africa and the Middle East from where US drone strikes in Yemen are launched … UK personnel are embedded at the base … it may be considered a small step to base or share assets with the US at Djibouti and therefore become party to the US covert drone war in Yemen."

Tom Watson told the Guardian: "The government is failing to engage on the core principles governing use of UK armed drones. This is worrying, given disclosures that UK personnel embedded at Camp Lemonnier are supporting US military operations and that UK Reapers won't be brought back to the UK.

"Our [all-party parliamentary group] complaint seeks the intervention of the UN special rapporteur to get to the bottom of this. We are entitled to know how the MOD proposes to use UK armed drones in the run-up to withdrawal from Afghanistan.'

A Human Rights Watch report has also reported that the Yemeni president, Abd Rabbuh Mansur, referred to the existence of a "joint operations room" including the US, the UK, Yemen, and Nato which "identifies in advance" the individuals who are "going to be targeted" [by US drone strikes].

In a letter to Reprieve, a senior MoD official denied the UK is providing "any military support to the US campaign of Remotely Piloted Aircraft System strikes on Yemen" adding that it was "unaware of any multinational control centre" of the sort reportedly described by the Yemeni president.

Reprieve's legal director Kat Craig said: "This is yet more evidence that the UK is deeply involved in the shady abuses of the 'war on terror.' The stationing of UK personnel at a base which has been identified as playing a key part in the secretive, unaccountable and illegal campaign of drone strikes in Yemen raises serious questions, as does the Yemeni president's statement that Britain is involved in targeting the strikes. The government must no longer dodge questions over its role in the secret drone war – it is high time it came clean with the British public."

On a visit to RAF Waddington last year, the defence secretary, Philip Hammond, was asked whether Reapers could be redeployed against al-Qaida supporters in Yemen. He replied: "We have to pursue the terrorists wherever they take themselves … wherever there's an ungoverned space, there's a risk."

Some British surveillance drones are already operating on anti-pirate patrols in the Red Sea and off the coast of East Africa. The Royal Navy's new unarmed, reconnaissance ScanEagle drone was first flown off a frigate on active service in the region since last December.

There has been a surge in US drone strikes against Al Qaida in the Arabian Peninsula (AQAP) this year. Around 30 people were reported to have been killed in a series of strikes in Yemen last month(APRIL) targeted against high profile leaders.

The MoD said: "UK personnel are not involved in the planning for, or operation of, any US Unmanned or Remotely Piloted Aircraft Systems (UAS/RPAS) from Camp Lemonnier."

The embedded British officers are not involved either in with liaising with Royal Navy surveillance drones on anti-piracy operations in the Red Sea, according to the MoD.

A MoD added: "The UK's use of RPAS in a combat role observes international humanitarian law and is governed by the same strict rules of engagement as those governing manned aircraft: the same attention is paid to avoiding civilian casualties, and the precision weapons UK RPAS carry can only be fired when commanded to do so by fully trained and qualified aircrews."