A scrappage programme ('cash-for-clunkers') that would provide US government vouchers to consumers to trade in their old cars looks uncertain.

Lawmakers had attached the programme to a must-pass bill to finance the wars in Iraq and Afghanistan, but talks on the broader legislation collapsed on Thursday over unrelated issues, Dow Jones said.

The programme that had been included in the war-funding bill closely resembled a bill passed by the US House of Representatives this week to provide vouchers of up to US$4,500 to consumers. Under that bill, trade-ins would have to get no higher than 18 mpg and have been built in 1984 or after. A $3,500 voucher would then be issued for the purchase or lease of a vehicle that got at least 22 mpg. The voucher would increase to $4,500 if the new vehicle was 10 mpg higher in fuel economy than the trade-in. Vouchers would be limited to vehicles under $45,000.

The standards would be looser for light-duty trucks, Dow Jones noted.

President Obama has called for Congress to pass a so-called cash-for-clunkers programme to stem a near-historic slide in new vehicle sales.

According to the report, the House bill faced opposition from several senators who said it wouldn't go far enough to reduce car emissions of greenhouse gases.

Critics argued it could end up providing vouchers toward the purchase of high-polluting Hummers and SUVs.

They pushed an alternative bill that they said would result in nearly a third more oil savings than the House bill. Their bill would set stricter fuel-economy requirements than the House version and allow vouchers to go toward used vehicles.