* Weaker demand in flood-hit regions drives down prices
* Softer demand seen in some cities due to G20 summit (Updates close prices)
SHANGHAI, July 25 (Reuters) - Chinese steel futures swung to losses after a moderate gain on Monday as cooling demand in flood-hit regions outweighed output cuts in a key industrial region.
Spot steel prices in some northern regions fell after the floods interrupted transportation and sales as well as hit demand, despite a mandated output cut in Tangshan, a key steel producing area, traders said.
"The positive news is a bigger output cut, particularly for coking plants, in Tangshan, but demand has also been hit by floods in some regions," said Yu Yang, an analyst with Shenyin & Wanguo Futures in Shanghai.
"The market could be volatile this week."
Benchmark prices for October rebar on the Shanghai Futures Exchange SRBcv1 traded slightly lower at 2,342 yuan ($350.68) a tonne by close.
Prices dropped 7 percent last week as investors booked profit on four straight weekly gains and worried over China's slowing property market and its impact on demand for steel products.
Seventy-two people have been killed and 78 are missing in Hebei province after rain triggered floods and landslides. In central Henan province, 15 people were killed and eight were missing, state media reported on Saturday. analysts also expected steel demand in the world's top producer to be hit by a slowdown and suspension at construction sites in regions near eastern China's Hangzhou city, where the next G20 leaders' summit will be held in early September.
Tangshan city in the top steel-producing province of Hebei has deepened mandated output cuts to improve air quality for a memorial day for a 1976 earthquake that killed at least 250,000 people.
The output cuts kicked off on July 12 but the pace has accelerated, traders said.
On the Dalian Commodity Exchange, the September iron ore contract DCIOcv1 rose 1 percent to 442.5 yuan. Coke DCJcv1 surged more than 4 percent amid supply shortage after the output cut while coking coal DJMcv1 rose more than 2 percent.
Iron ore for immediate delivery to China's Tianjin port .IO62-CNI=SI dropped 40 cents to $55.70 a tonne on Friday, according to The Steel Index. The index dropped 3.6 percent last week but has still jumped around 30 percent so far this year.
($1 = 6.6775 Chinese yuan)