Does the jury have to continue deliberating after the first 3 times that it has failed to reach a verdict?

2 Though it was not discussed in our opinion, the Government's trial brief in United States v. Williams noted that the trial court not only told the jury to continue deliberating the first three times that it indicated it was unable to reach a verdict but also gave the jury supplemental instructions "urging them to attempt to reconcile their differences of opinion if possible but cautioning them not to abandon their views merely to achieve unanimity." Brief and Appendix for the United States at 70, 71, United States v. Williams, 205 F.3d 23 (2d Cir. 2000).