A watchdog group says Sudan is expanding its capacity to conduct air strikes along the border with neighboring South Sudan.

The Satellite Sentinel Project says images from space show Sudan upgrading airbases it recently captured from rebels in Blue Nile state, which borders South Sudan.

It says the upgrades include newly-improved airstrips and new helicopter pads.

The report comes a day after Sudan was accused of dropping four bombs around a refugee camp in South Sudan's Unity State on Thursday.

A local South Sudanese official said the attack killed 12 people and wounded at least 20. Other reports said there were no casualties.

Sudan denies bombing South Sudan's territory.

The two countries have been almost constantly at odds since the south became independent in July.

On Thursday, South Sudan's President Salva Kiir denied accusations that his country backs rebels fighting Sudan's government in two border states, Blue Nile and Southern Kordofan.

Mr. Kiir also accused Sudanese President Omar al-Bashir of planning to invade and retake South Sudan.

Sudan has recognized the south's independence but the sides have yet to resolve issues over borders — especially in the oil-rich Abyei region — and the sharing of oil revenue. The south took over most of Sudan's oil, but but pipelines to the sea run through the north.