South Sudan has accused Sudan of building up forces along its disputed border, the site of clashes between the two countries last September.
South Sudan's defence ministry described the deployment of troops on Tuesday as "unusual" and said they were ready for a possible incursion.
"The last two months have seen an unusual build-up of forces along our common border with the Republic of Sudan," Majak D'Agoot, South Sudan's deputy defence minister, told reporters in the capital Juba, without giving any numbers.
"Our forces are in the state of maximum readiness to repel any attack by Khartoum. We will stay in our current positions, we will keep to the terms of the (September) agreement," D'Agoot said.
Sudan's army and foreign ministry spokesmen could not be immediately reached for comment.
The two countries came close to war last April in the worst border clashes since South Sudan seceded in 2011 under a peace agreement that ended one of Africa's longest civil wars.
"We are concerned again about this hawkish mindset, about the ruling elites in Khartoum who would want to escalate the situation along the border and possibly provoke a war between the two countries,"
- Majak D'Agoot, South Sudan's deputy defence minister
The African Union brokered a deal in September to defuse hostilities.
But the nations have failed to set up a demilitarised border zone and resume oil exports from the landlocked South Sudan through Sudanese pipelines, as agreed in Addis Ababa.
Such a buffer zone is a pre-condition for Sudan to allow oil exports to restart.
Juba shut down its output of 350,000 barrels a day a year ago in a row with Khartoum over pipeline fees.
D'Agoot said South Sudan had alerted other countries in the region, the African Union and the UN Security Council about what he called recent border violations by Sudan.
"We are concerned again about this hawkish mindset, about the ruling elites in Khartoum who would want to escalate the situation along the border and possibly provoke a war between the two countries," he said.
On Sunday, Sudan's state news agency SUNA said an infantry brigade had boosted security at the Heglig oilfield on the Sudan side of the disputed border. It was not clear if D'Agoot was referring to these troops.
Secure and stable
South Kordofan state governor Ahmed Haroun told SUNA the situation was secure and stable at the oilfield, which South Sudan's army briefly seized in April.
The countries are set to resume talks in Addis Ababa this month but diplomats expect no progress in setting up the border zone.
Two meetings between Sudan's President Omar Hassan al-Bashir and South Sudan's Salva Kiir last month failed to break a deadlock due to the deep mistrust between the nations, a legacy of the long civil war.
As well as getting oil flows restarted, both sides also need to decide on ownership of large strips of the almost 2,000 km long border.
South Sudan's army said on Saturday it had killed seven fighters from a rebel group supported by Sudan which had crossed into Upper Nile state.