Flood-hit parts of the nation were under threat of a "slow-motion disaster," forecasters warned Thursday as rivers swelled to historic levels and authorities closed a major Interstate.
At least 21 people have been killed in Missouri and Illinois and there have been widespread evacuations in the wake of a powerful winter storm. Two rivers have risen to record levels set in the so-called "Great Flood" of 1993.
Most of the deaths were in Missouri, where Gov. Jay Nixon on Tuesday declared a state of emergency and activated the national guard earlier this week.
Most of the rain moved out during Wednesday, but authorities warned the situation on the ground was still deteriorating early Thursday.
RELATED: Death toll rises in Missouri floods, threat not over
Officials closed Interstate 55 in both directions south of St. Louis to allow crews to deploy sandbags and pumps, Greg Horn, a district engineer at the Missouri Department of Transportation, told NBC News around 2 a.m. ET.
The highway was in danger of being overtaken by the Meramec River, close to where it runs into the Mississippi near the town of Arnold. Other roads, including Interstate 44 over the Meramec River, were also closed, according to The Associated Press.
The floods were the hangover from a large storm system that battered the center of the nation with tornadoes, ice and rain over the weekend. Despite the rain having dried up in the worst affected areas, most rivers were rising Thursday, swollen by floodwaters still flowing downstream.
Further north, in Missouri and Illinois, rivers and streams were expected to reach their highest on Thursday. While waterways further south, in Arkansas, Tennessee, Mississippi and Louisiana, were not expected to crest for a couple of days.
Scattered flood warnings from the National Weather Service were still in effect across the entire region.
"It's a slow-motion disaster," said Michael Palmer, lead forecaster at The Weather Channel. "A lot of these areas are going to be nearing record crests in the coming days."
Nine levees had already been topped by water by Wednesday, the U.S. Army Corps of Engineers said according to the AP. Although most of these levees were barriers made of earth protecting farmland rather than populated areas.
President Barack Obama called Gov. Nixon for a briefing Wednesday and directed his staff to coordinate on any federal assistance required.
"We are here before the water rises. We're here after it falls," Nixon said. "We're here until this place is back to where it was before. And sometimes that takes a while."
Meanwhile, officials named the five international soldiers temporarily stationed at Fort Leonard Wood, Missouri, who died Saturday when their vehicle was swept off the road in flash flooding.
The men, who were returning to their post from Osage Beach, Missouri, were: Maj. Mohammad Hassan Ibrahim, 32, from Egypt; Maj. Akram Abu Al-rub, 38, from Jordan; Capt. Ahmed Moussouni, 32, from Algeria; Capt. Ahmed Abdelghani, 29, from Egypt; and Capt. Hasman Hussin, 33, from Malaysia.
They were all on a program that brings foreign soldiers to the U.S. for training.
This article first appeared on NBCNews.com.