Seoul also announced additional restrictions on the movement of its citizens to the jointly-run Kaesong industrial park, just a few kilometres (miles) over the border inside North Korea. The South has taken an uncompromising stance in the wake of Wednesday's test, urging the international community to impose harsh sanctions on Pyongyang, and resuming high-decibel propaganda broadcasts into North Korea.
In a show of strength on Sunday, a B-52 Stratofortress -- flanked by South Korean F-15 fighter jets and US F-16 planes -- flew over Osan Air Base, some 70 kilometres (45 miles) south of the inter-Korean border.
The US military said the fly-by was a demonstration of the "ironclad" commitment to its military alliance with South Korea, and a direct response to the North's fourth nuclear test.
"South Korea and the US are in close consultations about additional deployment of other strategic assets on the Korean Peninsula," Defence Ministry spokesman Kim Min-Seok told a regular press briefing in Seoul.
US and South Korean media reports have speculated that the nuclear-powered aircraft carrier US Ronald Reagan -- currently based in Japan -- B-2 stealth bombers and F-22 stealth fighter jets, are among the deployments being considered. Under the US-South Korea military alliance, there are nearly 30,000 US troops permanently stationed in South Korea, which is also protected by the US "nuclear umbrella".
North Korea claims last week's test was of a miniaturised hydrogen bomb -- a claim largely dismissed by experts who argue the yield was far too low for a full-fledged thermonuclear device. The test has been widely condemned and the UN Security Council is discussing a new resolution that would tighten sanctions already imposed on the North in the wake of its three previous nuclear tests and banned missile launches.
As well as resuming the propaganda broadcasts, which one top North Korean official warned could bring the peninsula to the "brink of war", Seoul has also taken measures to restrict movement to the Kaesong industrial park.
On Monday, the Unification Ministry announced that the number of South Koreans allowed to stay overnight in Kaesong was being reduced from 800 to 650.