Emergency crews scrambled to bolster the banks of the Tisza River that flows into Serbia from Hungary after it reached its highest levels in parts of the two countries already hit by the flooding.

Further downstream in Romania, the rising Danube forced authorities to evacuate more than 4,700 people, mainly in the southern towns of Calarasi and Fetesti.

In neighbouring Bulgaria, thousands of volunteers joined army troops to strengthen embankments along the surging Danube with sandbags, as the government came under political pressure for its handling of the disaster.

The situation was expected to worsen across the Balkans within hours, with heavy rainfall forecast later Tuesday or Wednesday, adding to the melting snow which has contributed to the flooding.

The Tisza River rose to a record 9.8 metres (32 feet) in the Hungarian town of Szeged, and was moving south towards the already bursting Danube, which flows on through Serbia, Romania and Bulgaria.

"About 158,000 people in 51,150 homes are threatened by the Tisza," said Tibor Dobson, spokesman for Hungary's national disaster prevention agency.

"High water is threatening 254 communes, 523 people have been evacuated and over 600 are working on reinforcing dykes in the town of Szeged," he added.

Downstream in Serbia, where the river is known as the Tisa, the floodwaters are expected to rise by another 40 centimetres after already having reached the highest recorded level of 9.2 metres (30 feet) near the town of Novi Knezevac.

"We are reinforcing embankments, increasing their height along 22 kilometres of the Tisa ... the highest levels are expected in the next two or three days," said Branislav Radanovic of northern Vojvodina province's water authority.

"There is a danger of possible breach of the embankments" around the village of Mosorin, which could endanger some 6,000 hectares of cropland and numerous fish farms, he added.

Before the latest threat from the Tisa, officials in Serbia had estimated the flooding of the fertile plains north of Belgrade would have a heavy financial impact on the country's crucial agricultural sector.

Some 220,000 hectares (544,000 acres) of farmland have been submerged in Vojvodina, considered Serbia's breadbasket.

Serbia's largest agricultural company, the PKB, has predicted major losses because of the flooding, said its director Mirko Martinovic.

Meanwhile, Serbian epidemiologists issued a warning about the possible spread of disease once the floodwaters subside, when rodents and insects thrive.

As the work on the dykes intensified in Bulgaria, the opposition prepared a parliamentary no-confidence motion against the government, alleging it failed to cope with previous disaster situations.

"The major threat does not come from overflowing but from a breakthrough in the anti-flood defences that have survived continuous high water pressure for a month now," a Bulgarian expert said.

Some 2,000 volunteers had filled more than 90,000 sandbags to stem breaks in dykes, the civil protection agency said, as water levels at the far northwestern Bulgarian town of Vidin reached 9.7 metres.

Downstream, the low-lying villages of Botevo and Simeonovo remained completely cut off as underground water oozed up to submerge houses and roads.

In the town of Nikopol, 40 percent of the riverside streets, homes and industrial buildings remained under water, as authorities rushed to construct makeshift plank bridges and restore electricity.

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