Oil that has washed ashore on several southern Japanese islands is likely from an Iranian tanker that sank in the East China Sea last month, Japan’s coast guard said Friday.

S Oil どんな→[that]
     V [has washed]
O ashore どんな→[on] (several southern Japanese) islands

打ち寄せたオイル

(likely) [from] (an Iranian) tanker
どんな [that]
V [sank]
沈んだタンカーからの

補足
[in] the East China Sea (last month),

東シナ海にて

S Japan’s coast guard
V [said] (Friday.)

海岸警備隊が言いました。