Image copyright AFP/Getty Images Image caption The PM has been in Romania as part of his EU renegotiation tour

The current level of net migration to the UK is "not sustainable", David Cameron has said, as he continued his diplomatic push for EU benefits reform.

The PM wants to curb migrants' benefits to cut immigration, but some European leaders are said to oppose the move.

After talks in Bucharest with the Romanian president, Mr Cameron said he was "confident" of reaching a deal on his four key renegotiation aims.

The UK PM is also visiting Poland on his two-day trip to eastern Europe.

Mr Cameron will hold talks with his Polish counterpart, Beata Szydlo, in Warsaw on Thursday.

Welfare reform

David Cameron has promised to hold an in-out vote on Britain's membership of the EU before the end of 2017, once his reform negotiations are concluded.

He is seeking a "new settlement" for the UK in the EU, made up of four key strands:

Protection of the single market for Britain and other non-euro countries

Boosting competitiveness by setting a target for the reduction of the "burden" of red tape

Exempting Britain from "ever-closer union" and bolstering national parliaments

Restricting EU migrants' access to in-work benefits such as tax credits

Of those objectives, European Council President Donald Tusk has said welfare reform is the most "delicate" and that there are "substantial political differences" over the plan.

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'Pressures'

Following talks with Romanian President Klaus Iohannis on Wednesday as part of his renegotiation tour, Mr Cameron said "good progress" was being made on the changes he is seeking, but acknowledged that "some areas are more difficult than others, particularly... on welfare".

He said he supported the principle of free movement to work, saying it was "a basic treaty right and a key part of the single market" and that Romanians and other Europeans "make a valuable contribution" to the UK.

Image copyright AFP/Getty Images Image caption Mr Tusk has written to EU leaders warning that uncertainty over the UK's future in the EU was "destabilising"

"But," he added, "it was never envisaged that free movement would trigger quite such vast numbers of people moving across our continent.

"And countries have got to be able to cope with all the pressures that can bring - on our schools, our hospitals and other public services."

He continued: "Net migration in the UK is running at well over 300,000 a year and that is not sustainable.

"So we do need to find ways to allow member states to make changes to their social security systems that will help them to deal with this issue."

Image copyright PA Image caption Mr Cameron's bid to curb EU migrants' in-work benefits is proving the most controversial

The PM said there would be "substantive discussion" on the UK's objectives at the December summit, "and I am confident we can find solutions in each area".

He said the EU "has shown before it has the flexibility to respond to the concerns of its member states" and "it needs to do so again".

Mr Cameron has conceded a deal will not be reached on the UK's objectives before Christmas. But Mr Tusk has said it should "pave the way" for a deal by February.