LONDON (Reuters) - British Foreign Secretary William Hague said on Monday that there would be pressure to intensify sanctions on Iran if it could not reach a deal with world powers over its disputed nuclear program.

But if Tehran could strike a preliminary agreement, world powers would lift some of the sanctions they have imposed on it offering it "limited, proportionate sanctions relief," Hague added, saying he felt there was a real chance of getting a deal.

Hague was speaking after Iran and six world powers, including Britain, came close to a preliminary agreement about Tehran's nuclear program in Geneva at the weekend.

"It's very important for the Iranian authorities to understand that the pressure will be there for greater sanctions, for an intensification of sanctions, unless an agreement is reached on these matters," Hague told parliament.

Playing down reports that France had blocked a deal, Hague said world powers had presented a united front to Iran at the end of the discussions. Britain wants an interim agreement with Iran as a first step towards a full deal on Tehran's nuclear program, he added.

In a sign of improving relations between London and Tehran, Britain's Foreign Office said earlier it had appointed a non-resident charge d'affaires to try to improve diplomatic ties after its Iranian embassy was ransacked in 2011.

(Reporting by Peter Griffiths; Editing by Andrew Osborn)