MOSCOW, July 3 (Reuters) - The rouble firmed against the dollar on Wednesday, trading in its narrowest range since new U.S. sanctions on Russia's key financial systems brought a halt to exchange trading in the dollar and euro in Moscow last month.

By 0757 GMT, the rouble was 0.5% higher at 87.40 against the dollar, trading largely in the 87-88 range, a relatively small band following weeks of volatility.

Sanctions on Moscow Exchange and its clearing agent, the National Clearing Centre (NCC), have led to a range of varying prices and spreads as trading shifted to the over-the-counter (OTC) market on June 14, obscuring access to reliable pricing for the Russian currency.

The average dollar-rouble mixed composite rate, calculated by LSEG and based on data from international brokers and counterparties, stood at 87.70.

Against the yuan, which had already become the most traded foreign currency in Moscow before the latest sanctions were imposed, the rouble shed 0.5% to 11.88, according to an analysis of the OTC market. It was down 0.4% at 94.80 against the euro .

We expect the rouble to trade in the 85-92 range to the dollar in July, said Mikhail Vasilyev, chief analyst at Sovcombank.

High rouble interest rates, a rise in oil prices, Russia's trade surplus, mandatory foreign currency sales for exporters and payment problems causing lower imports all play in favour of the rouble, Vasilyev said, while sanctions risks, capital outflow and seasonally higher foreign currency demand hamper its strengthening.

The market is looking ahead to the finance ministry's FX interventions plan for the month ahead, due to be announced at 0900 GMT.

Brent crude oil, a global benchmark for Russia's main export, was up 0.4% at $86.58 a barrel. (Reporting by Alexander Marrow; Editing by Anil D'Silva)