Copper prices climbed slightly on Wednesday, supported by ongoing disruption to mine supply and a softer dollar, as a U.S. government shutdown began.

Three-month copper on the London Metal Exchange was up 0.4% at $10,307 per ton in official open outcry trading. Markets in top metals consumer China are closed from October 1 to 8 for the National Day holiday.

The absence of Chinese market participants "could lead to increased volatility over the coming days, with market appetite appearing to be skewed to the upside," Sucden Financial said in a note.

Last week's force majeure by Freeport-McMoRan at its Grasberg mine in Indonesia acted as an incentive to break away from rangebound conditions, but any significant price upside may be tempered by a key resistance level of $10,500 a ton, the brokerage added.

Potentially exacerbating the Grasberg outage, the union for supervisors at Antofagasta's Los Pelambres copper mine in Chile has rejected a new contract offer, paving the way for a potential strike, a union leader said on Tuesday.

Top copper producer Chile's output fell 9.9% year-on-year in August, the sharpest drop in more than two years, after an accident at Codelco's flagship mine at the end of July.

Base metal prices also found support from a weaker dollar , which sank to a one-week low against major currencies on Wednesday as the U.S. government shutdown unsettled markets. A weaker dollar makes metals more affordable for holders of other currencies.

In the rest of the base metals complex, aluminium rose 0.3% to $2,687.50 a ton, zinc gained 0.2% to $2,966.50, and lead was up 0.3% to $1,994.50. Nickel slipped 0.5% to $15,160, while tin gained 1.5% to $35,950, after earlier hitting $36,090, its highest since April 4.

(Reporting by Tom Daly; additional reporting by Lucas Liew; Editing by Harikrishnan Nair, Subhranshu Sahu, Shailesh Kuber and Shreya Biswas)