Turkey's president says a full withdrawal of its forces from divided Cyprus is "out of the question" unless Greece also agrees to pull out troops.
Hopes of reunification have been raised after talks between Turkish and Greek Cypriots in Geneva this week.
But a number of stumbling blocks remain, including whether any Turkish troops would stay in northern Cyprus after reunification.
The island's communities have been split since 1974.
Key obstacles to a deal include the return of property to tens of thousands of Cypriots who fled their homes in 1974, as well as the presence of troops.
Turkey still has 30,000 troops stationed in the island's north, whose presence Greece opposes. On Friday, Greek Cypriot President Nicos Anastasiades reiterated a call for all Turkish troops to leave. Greece is thought to have about 1,000 troops stationed on the island.
Turkish Cypriot leader Mustafa Akinci said one side demanding that the other remove its troops would rule out a mutually acceptable solution.
On Thursday, Turkish Foreign Minister Mevlut Cavusoglu had said discussions were "at a critical juncture" and that technical experts would now hold talks.
The meetings are likely to begin on 18 January, with a focus on thorny security issues, reports say.
What are the sticking points?
Property: What should happen to the properties that Greek Cypriots had to abandon in 1974? Should they get the right to take their old homes back, or be compensated - and if so by how much?
Security: How can the security of the Turkish Cypriots be guaranteed if Turkey's estimated 30,000 troops leave? Greek Cypriots see them as an occupying force, so should some stay or should Turkey retain the right to intervene? Who would act as a guarantor of the deal? The EU, of which Cyprus is already a member, or the UK, which has two military bases on the island?
Power and the role of the EU: There is talk of a rotating presidency, but how would that work? And could a Turkish Cypriot president really represent the country from time-to-time at EU summits?
Territory: How much more territory should Greek Cypriots gain to reflect the fact that they make up the majority of the island's population? UN peacekeeping forces estimate that 165,000 Greek Cypriots fled or were expelled from the north, and 45,000 Turkish Cypriots from the south, although the parties to the conflict say the figures are higher.
The end goal is for the Greek Cypriot and Turkish Cypriot communities to share power in a two-state federation.
Any deal would have to win the support of both Cypriot communities in separate referendums.
While diplomats have made positive noises on the progress being made, Mr Erdogan said that Greece and the Cypriot government "still have different expectations" from Turkey.
Another obstacle, he added, was on a possible rotating presidency on Cyprus.
Instead of seeing four Greek Cypriot presidencies for every one by Turkish Cypriots, Mr Erdogan said he wanted a 2:1 balance in Greece's favour instead.
- 1955 - Greek Cypriots seeking unification with Greece begin guerrilla war against British rule
- 1960 - Independence from British rule leads to power-sharing between Greek Cypriot majority and Turkish Cypriot minority
- 1963/64 - Inter-communal violence
- 1974 - Cypriot President, Archbishop Makarios, deposed in a coup backed by Greece's military junta - Turkey sends troops to the island, who then occupy a third of it in the north
- 1983 - Rauf Denktash declares breakaway Turkish Republic of Northern Cyprus, recognised only by Turkey
- 2004 - Cyprus, still divided, joins the EU, after a UN peace plan was backed by Turkish Cypriots but rejected by Greek Cypriots