UK efforts to tackle the migration crisis in the Mediterranean are "some distance away" from having a measurable impact, the independent aid watchdog has found.
The watchdog also raised concerns that some programmes risked causing "unintended harm to vulnerable migrants", particularly in Libya, one of the main embarkation points for migrants risking the treacherous sea crossing.
The Independent Commission for Aid Impact (ICAI) also criticised as unhelpful a tendency by the Government to rebrand existing aid programmes as migration-related when they were not designed with that aim.
The ICAI report, which focused on work in Ethiopia, Libya and Nigeria, found that the Government had moved quickly to build up capacity on migration-related programmes since 2015.
But in Libya and Nigeria, migration-related aid programming was "small and rather fragmented".
"The programmes also remain some distance away from making a measurable impact on irregular migration through the central Mediterranean," the report noted.
"This is the case partly because UK aid's focus on migration is very recent."
In Ethiopia, where "thinking on irregular migration is most advanced", there were promising signs including an £80 million "jobs compact" programme aimed at creating work for refugees - giving them a reason to stay in Africa rather than heading to Europe.
Dr Alison Evans, ICAI's chief commissioner who led the review, said: "Global migration is increasingly an important issue for UK aid, but in addressing this the Government faces a complex and rapidly changing context, and a range of difficulties.
"The forthcoming jobs compact in Ethiopia, which aims to create employment opportunities for refugees and host communities, is promising, and could make a positive impact.
"However, we are concerned that some UK aid programmes have unhelpfully been re-labelled as 'migration-related' when there is little evidence they will reduce irregular migration.
"We are also concerned about the risk of unintended harm to vulnerable migrants, particularly in difficult operating contexts such as Libya, and we have urged the Government to do more to identify and manage these risks.
"UK aid's focus on irregular migration is at its early stages, but at the moment our review found it is some distance from making a measurable impact in the central Mediterranean."
A Government spokesman said: "Cross-government efforts are tackling the root causes of migration by building opportunity and stability for people in their home regions so they don't need to make the perilous journey across the Mediterranean.
"ICAI rightly praises our innovative work in Ethiopia where we have created 30,000 jobs for refugees.
"We're also getting help to vulnerable migrants who have already started their treacherous journey.
"Since May 2015, British vessels have saved more than 13,000 lives in the Mediterranean."