Export AVAsset while writing to disk?

I am trying to export the last X seconds of a video being recorded to a seperate file. So a whole video is being written to disk, and snippets of length X seconds can also be written. However, I am having trouble exporting the AVAsset that is being written to, I believe because the asset is not marked as exportable (the precondition fails). If I remove the precondition, then I receive the error The operation could not be completed. How can I accomplish this?

func trimVideoAndSaveToFile(videoURL: URL, startSeconds: Double, endSeconds: Double, outputURL: URL) {
    print("trim video!")
    let asset = AVURLAsset(url: videoURL)
    precondition(asset.isExportable)

    let exportSession = AVAssetExportSession.init(asset: asset, presetName: AVAssetExportPresetHighestQuality)!

    exportSession.outputURL = outputURL
    print(outputURL)
    exportSession.shouldOptimizeForNetworkUse = true
    exportSession.outputFileType = AVFileType.mov
    let start = CMTimeMakeWithSeconds(startSeconds, preferredTimescale: 600)
    let end = CMTimeMakeWithSeconds(endSeconds, preferredTimescale: 600)
    let duration = CMTimeSubtract(end, start)
    let range = CMTimeRangeMake(start: start, duration: duration)
    exportSession.timeRange = range
    exportSession.exportAsynchronously {
        switch(exportSession.status) {
        case .completed:
            print("finished recording to \(outputURL)")
            break
        //
        case .failed:
            print("change recording failed! Reason: \(String(describing: exportSession.error?.localizedDescription))")
            break
        //
        case .cancelled:
            print("change recording cancelled!")
            break
        //
        default: break
        }
    }
}

videoURL is the url used in self.fileOutput.startRecording(to: url, recordingDelegate: self)

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