After six week of technical and then high-level political talks, Iran and the 5+1 Powers (US, Britain, Germany, France, China, and Russia) have agreed a start date of January 20 for November’s interim nuclear deal.

The accord was announced in Tehran by Deputy Foreign Minister Abbas Araqchi and welcomed in Washington by President Obama.

Under the arrangement, Iran will suspend enrichment of uranium to 20% and convert much of its existing stock to fuel plates, which have no potential for military use. The 5+1 Powers will unblock $4.2 billion of Iranian assets in eight payments over the deal’s six months — the first release coming on February 1.

Araqchi also said all sanctions on Iran’s automotive and petrochemical sectors and on transfer of gold and precious metals — used by countries like Turkey to pay for Iranian oil and natural gas — will be lifted from January 20.

Iranian officials had said earlier this month that implementation would start January 20, but Araqchi then pointed to “unresolved political issues”. As late as last Thursday — with Araqchi meeting the senior negotiators of the US and the 5+1 Powers — the Supreme Leader was denouncing US “hostility” and deceit.

Sunday’s announcement may check some of that scepticism in Tehran, but in Washington, the US Congress is still threatening to pass a bill mandating new sanction on Iran if they believe the Islamic Republic is not fulfilling terms of the deal.

Perhaps because of this, Araqchi was immediately cautious about the prospect of a comprehensive, final agreement by January 2014:

Talks on final step will be difficult with difficult issues. Reaching final agreement unclear….

Obama said the upcoming talks have a 50-50 chance of success, I say it’s even less.

Araqchi warned, “If the US congress wants to pressure us on new pretexts, we’d say with certainty that we won’t negotiate under pressure at all, and if new sanctions are imposed, the Geneva deal would be canceled.”