The EU has hailed a plunge in the number of migrants crossing Niger to reach Europe as a success for its anti-trafficking strategy, even as MEPs complained that the bloc’s figures did not add up.

The volume of migrants making the perilous journey across Niger’s desert fell from 70,000 in May to 1,500 in November, according to an EU statement released on Wednesday.

The EU’s foreign policy chief, Federica Mogherini, said the bloc’s funding of anti-migration measures in Africa was creating “positive results and important building blocks for new cooperation”.

Corruption stymies Niger's attempts to stem flow of migrants to Europe Read more

The Finnish prime minister, Juha Sipila, has called for the scheme to be extended to Iraq, Afghanistan and Somalia. At an EU summit on Thursday, an announcement of thousands more “voluntary assisted returns” to Libya is expected.

But doubts about the probity of the figures for Niger cast a shadow over Wednesday’s announcement. The declared drop in migrants crossing Niger is based on an assessment of just two monitoring points, and data that “does not reflect overall entries or exits to and from Niger”.

That assessment – by the International Organisation for Migration – also suggests that migrants are avoiding key towns and transit points in Niger “for fear of repression”, casting further doubt on what EU officials had described as “a success story” for EU policy.

The Green MEP Judith Sargentini branded the European commission’s claim to have reduced migrant numbers in Niger as being “at least questionable, if not harmful for migrants”.

She said: “EU support for Niger’s border guards has made people smuggling even more profitable, as the prices of people smuggling rise due to increased border guards. Meanwhile, proposals for legal pathways for refugees and economic migrants are nowhere to be seen. In all aspects, this is a failing approach.”

Facebook Twitter Pinterest Migrants holding water containers sit on the back of an overcrowded truck at a local immigration transit centre in Agadez. Photograph: Akintunde Akinleye/Reuters

Many migrants are thought to have travelled north through Niger to Libya, from where human smugglers ferry them to Italy on ramshackle boats.

Sargentini, a member of the European parliament’s development committee, pointed out that more people had died in the Mediterranean Sea this year than in 2015, and said that the EU’s pride at trends in Niger was misplaced. “If the EU really did reduce the number of people transiting through that country, where are those people now, and what state are they in?” she asked.

Senior EU officials insist it is too early to conclude that traffickers have diverted their routes via neighbouring Mali to avoid detection by EU-trained border guards. But one said that the bloc was “starting to see circumstantial evidence” of this.

By early next year, the EU wants to sign agreements linking aid to migration restrictions with five African countries – Niger, Nigeria, Mali, Senegal and Ethiopia. EU officials will then be deployed in these countries to help with measures such as the biometric registration of migrants passing through.

In a progress report for Senegal, EU sources say “a lot of work has to be done on the root causes of migration”, although they are hopeful of progress in Nigeria, where the readmission of deported EU migrants began in October.

In total, 4,430 people have been repatriated from Europe under the migration pacts so far, and a further 2,700 people have been returned to priority countries.

However, Raphael Shilhav, Oxfam’s EU migration adviser, said the policy had been short-sighted in its depiction of migration as a negative phenomenon. “Migration is an important tool for development in western Africa and a vital coping mechanism for displaced peoples,” he said. “In the Lake Chad basin, we have millions of people displaced – including across borders – and an EU intervention to close down borders is problematic. These people need humanitarian assistance.”

Many NGOs and MEPs complain that existing development aid pledges are being diverted towards projects aimed at stemming migration. There are no reliable figures for how much of the contribution that EU countries make to a €2.5bn Africa Trust fund linked to migration curbs comes from existing development aid budgets.

“You can’t say that it is new [money],” said one EU official. “It may be additional funding that is being mobilised for this purpose.”