Local authorities in Valencia have extended travel restrictions for another two days to facilitate the work of the emergency services, cancelled school classes and urged people to work from home.
Gen Marcos said gridlock on some roads was making it hard to distribute food to the 69 affected municipalities.
He said that the number of troops on the ground had been “scaled up” – with 7,800 due by 20:00 local time (19:00 GMT) on Monday. These were being supported by 17,000 volunteers and 5,000 army soldiers offering logistical support, he said.
A warship carrying 104 marine infantry soldiers as well as trucks with food and water docked in Valencia’s port on Monday.
Gen Marcos said the UME was “doing everything we can”, including working double shifts.
He added: “We must be disciplined and we must be patient. I know that’s hard, because pain and emotion means that’s not easy.”
In Catalonia, the Ministry for Ecology said that the State Meteorological Agency (Aemet) had issued a red alert due to the “extreme danger” of torrential rains.
Aemet is warning that up to 8cm of rain could fall across coastal areas into Tuesday morning – but that localised downpours could lead to “very different” accumulations between nearby towns.
The flooding in Valencia was caused by a Dana phenomenon – when warm, moist air meets cold air, creating an unstable weather system.
Scientists say the effects of climate change made the floods worse.